@@ -366,13 +366,16 @@ def _score_html_impl(ctx):
     ]
     merge_inputs = [sphinx_html_output]
 
-    # Add each dependency
-    for dep in ctx.attr.deps:
-        if SphinxModuleInfo in dep:
-            dep_html_dir = dep[SphinxModuleInfo].html_dir
-            dep_name = dep.label.name
-            merge_inputs.append(dep_html_dir)
-            merge_args.extend(["--dep", dep_name + ":" + dep_html_dir.path])
+    # Every module transitively required by this one, each contributing its
+    # own_html_dir (never another module's already-merged html_dir), so each
+    # lands exactly once, at depth 1, regardless of how many paths reach it
+    # in a diamond dependency graph (see SphinxModuleInfo.transitive_modules).
+    transitive_modules_from_deps = depset(
+        transitive = [dep[SphinxModuleInfo].transitive_modules for dep in ctx.attr.deps if SphinxModuleInfo in dep],
+    ).to_list()
+    for module in transitive_modules_from_deps:
+        merge_inputs.append(module.own_html_dir)
+        merge_args.extend(["--dep", module.name + ":" + module.own_html_dir.path])
 
     # Auto-detect static files from srcs: any file whose short_path contains
     # '/_static/' is a static asset that Sphinx may not copy correctly in the
@@ -400,6 +403,13 @@ def _score_html_impl(ctx):
         DefaultInfo(files = depset([html_output])),
         SphinxModuleInfo(
             html_dir = html_output,
+            own_html_dir = sphinx_html_output,
+            # Reuses the already-flattened transitive_modules_from_deps list
+            # collected above for the merge step, plus this module itself.
+            transitive_modules = depset(
+                [struct(name = ctx.label.name, own_html_dir = sphinx_html_output)] +
+                transitive_modules_from_deps,
+            ),
         ),
         OutputGroupInfo(
             sphinx_sources = depset([config_file] + sphinx_source_files),
 
@@ -258,7 +258,21 @@ SphinxIndexFileInfo = provider(
 SphinxModuleInfo = provider(
     doc = "Provider for Sphinx HTML module documentation",
     fields = {
-        "html_dir": "Directory containing HTML files",
+        "html_dir": "Directory containing this module's HTML, merged with its " +
+                    "transitive dependencies (what sphinx_html_merge.py produces).",
+        "own_html_dir": "Directory containing only this module's own HTML, before " +
+                        "any dependency is merged in. What the flat merge (see " +
+                        "transitive_modules) copies from — each transitive module " +
+                        "contributes its own_html_dir, never another module's " +
+                        "already-merged html_dir, so a diamond dependency doesn't " +
+                        "get copied once per path to it.",
+        "transitive_modules": "Depset of struct(name, own_html_dir), one entry per " +
+                              "module transitively required by this one, including " +
+                              "this module itself. Self-inclusive union: each dep's " +
+                              "own transitive_modules already contains itself, so " +
+                              "unioning deps' depsets yields the full flat closure " +
+                              "without this module needing to add each dep " +
+                              "individually on top.",
     },
 )
 
 
@@ -15,9 +15,14 @@
 """Merge multiple Sphinx HTML output directories.
 
 This script merges Sphinx HTML documentation from multiple modules into a single
-output directory. It copies the main module's HTML as-is, and then copies each
-dependency module's HTML into a subdirectory, excluding nested module directories
-to avoid duplication.
+output directory. It copies the main module's HTML as-is, then copies every
+transitively required module's *own* (unmerged) HTML into a subdirectory --
+flat, one level deep, one copy per module regardless of how many dependency
+paths reach it. sphinx_module.bzl is responsible for passing --dep as the
+transitive closure (SphinxModuleInfo.transitive_modules), with each --dep
+pointing at that module's own_html_dir rather than its (possibly
+further-merged) html_dir -- this script has no way to tell the two apart and
+would duplicate a diamond dependency if handed the latter.
 
 Usage:
     sphinx_html_merge.py --output OUTPUT_DIR --main MAIN_HTML_DIR [--dep NAME:PATH ...]
@@ -94,10 +99,12 @@ def copy_html_files(src_dir, dst_dir, is_dependency=False, sibling_modules=None)
                      dropped (the merged site uses one shared _static/ at the
                      root) and their internal links rewritten for the new
                      nesting depth.
-        sibling_modules: Set of sibling module directory names to skip (so nested
-                        copies of other modules already merged elsewhere aren't
-                        duplicated) and to rewrite intra-site links for. Only
-                        meaningful when is_dependency is True.
+        sibling_modules: Set of other module directory names to rewrite intra-site
+                        links for (e.g. href="other_module/..." needs a "../" prefix
+                        added for the new nesting depth). Only meaningful when
+                        is_dependency is True. src_dir is always a module's own,
+                        unmerged HTML now (never another module's already-merged
+                        tree), so there is nothing nested under it to skip.
     """
     src_path = Path(src_dir)
     dst_path = Path(dst_dir)
@@ -175,9 +182,6 @@ def copy_tree(src, dst, rel_path):
                 # Never publish Sphinx's own build cache.
                 if item.name in BUILD_ARTIFACT_DIRS:
                     continue
-                # Skip nested copies of sibling modules to avoid duplication.
-                if item.name in sibling_modules:
-                    continue
                 # Dependencies use the merged site's shared _static/ instead
                 # of their own.
                 if is_dependency and item.name in (
@@ -217,15 +221,14 @@ def merge_html_dirs(output_dir, main_html_dir, dependencies, extra_static=None):
         shutil.copy2(src_file, dst)
         logging.info("Copied extra static %s → _static/%s", src_file, dest_subpath)
 
-    # Collect all dependency names for link fixing and exclusion
+    # Collect all dependency names for intra-site link rewriting.
     dep_names = [name for name, _ in dependencies]
 
     # Then copy each dependency into a subdirectory with link fixing
     for dep_name, dep_html_dir in dependencies:
         dep_output = output_path / dep_name
         logging.info("Copying dependency %s from %s to %s", dep_name, dep_html_dir, dep_output)
-        # Exclude other module directories to avoid nested modules
-        # Remove current module from the list to get actual siblings to exclude
+        # Other modules in this merge, to rewrite intra-site links for.
         sibling_modules = set(n for n in dep_names if n != dep_name)
         copy_html_files(
             dep_html_dir,
 
@@ -77,6 +77,7 @@ load(
     "score_toolchain_override_test",
     "sphinx_module_info_fields_test",
     "sphinx_module_providers_test_suite",
+    "transitive_modules_dedup_test",
     "two_phase_html_second_test",
     "two_phase_needs_first_test",
 )
@@ -148,17 +149,32 @@ toolchain(
 # Test 1: Multi-Module Aggregation
 # Dependency graph: module_a_lib -> module_b_lib -> module_c_lib
 #                   module_a_lib -> module_c_lib (also direct)
+# module_d_lib is a diamond dependency reachable only transitively, via
+# module_b_lib AND module_c_lib (never directly from module_a_lib) -- a
+# regression fixture for the HTML-merge flattening: module_d_lib's HTML must
+# be merged into module_a_lib's published site exactly once, not once per
+# path that reaches it.
+sphinx_module(
+    name = "module_d_lib",
+    srcs = glob(["fixtures/module_d/*.rst"]),
+    index = "fixtures/module_d/index.rst",
+)
+
 sphinx_module(
     name = "module_c_lib",
     srcs = glob(["fixtures/module_c/*.rst"]),
     index = "fixtures/module_c/index.rst",
+    deps = [":module_d_lib"],
 )
 
 sphinx_module(
     name = "module_b_lib",
     srcs = glob(["fixtures/module_b/*.rst"]),
     index = "fixtures/module_b/index.rst",
-    deps = [":module_c_lib"],
+    deps = [
+        ":module_c_lib",
+        ":module_d_lib",
+    ],
 )
 
 sphinx_module(
@@ -878,6 +894,17 @@ sh_test(
     tags = ["manual"],
 )
 
+# End-to-end regression test: module_d_lib (a diamond dep of both
+# module_b_lib and module_c_lib, never directly of module_a_lib) must be
+# merged into module_a_lib's published site exactly once, flat, not once
+# per dependency path that reaches it.
+sh_test(
+    name = "module_d_dedup_test",
+    srcs = ["check_module_d_dedup.sh"],
+    data = [":module_a_lib"],
+    tags = ["manual"],
+)
+
 # ============================================================================
 # SEooC-Specific Tests
 # ============================================================================
@@ -1092,6 +1119,14 @@ sphinx_module_info_fields_test(
     target_under_test = ":module_a_lib",
 )
 
+# Regression test for the diamond-dependency HTML-merge flattening:
+# module_d_lib is a dep of both module_b_lib and module_c_lib (never
+# directly of module_a_lib), so it's reachable via two paths.
+transitive_modules_dedup_test(
+    name = "transitive_modules_dedup_test",
+    target_under_test = ":module_a_lib",
+)
+
 score_needs_info_fields_test(
     name = "score_needs_info_fields_test",
     target_under_test = ":module_a_lib_needs",
@@ -1395,6 +1430,7 @@ test_suite(
         ":test_sphinx_html_merge",
         ":test_sphinx_module_ext",
         ":test_trlc_rst_image_rendering",
+        ":transitive_modules_dedup_test",
         ":unit_component_tests",
         "//fixtures/image_srcs:requirements_image_tests",
     ],
 
@@ -0,0 +1,46 @@
+#!/bin/bash
+# *******************************************************************************
+# Copyright (c) 2026 Contributors to the Eclipse Foundation
+#
+# See the NOTICE file(s) distributed with this work for additional
+# information regarding copyright ownership.
+#
+# This program and the accompanying materials are made available under the
+# terms of the Apache License Version 2.0 which is available at
+# https://www.apache.org/licenses/LICENSE-2.0
+#
+# SPDX-License-Identifier: Apache-2.0
+# *******************************************************************************
+set -euo pipefail
+
+# End-to-end regression test for the diamond-dependency HTML-merge
+# flattening: module_d_lib is a dep of both module_b_lib and module_c_lib
+# (never directly of module_a_lib), so before the fix its HTML was copied
+# once per path that reached it -- nested under module_b_lib/module_d_lib/
+# AND module_c_lib/module_d_lib/, never at the top level. After the fix it
+# must land exactly once, flat, at the top level of the merged site.
+# TODO: pass the HTML dir via args instead of using a hardcoded relative path,
+# e.g. args = ["$(rootpath :module_a_lib)"] in the sh_test.
+html_dir="./module_a_lib/html"
+
+if [[ ! -d "$html_dir" ]]; then
+    echo "Error: Directory not found: $html_dir" >&2
+    exit 1
+fi
+
+if [[ ! -f "$html_dir/module_d_lib/index.html" ]]; then
+    echo "Error: Expected flat, top-level $html_dir/module_d_lib/index.html not found" >&2
+    exit 1
+fi
+
+if [[ -d "$html_dir/module_b_lib/module_d_lib" ]]; then
+    echo "Error: module_d_lib was duplicated, nested under module_b_lib/" >&2
+    exit 1
+fi
+
+if [[ -d "$html_dir/module_c_lib/module_d_lib" ]]; then
+    echo "Error: module_d_lib was duplicated, nested under module_c_lib/" >&2
+    exit 1
+fi
+
+echo "✓ module_d_lib's HTML appears exactly once, flat, in the merged output"
@@ -0,0 +1,45 @@
+..
+   # *******************************************************************************
+   # Copyright (c) 2026 Contributors to the Eclipse Foundation
+   #
+   # See the NOTICE file(s) distributed with this work for additional
+   # information regarding copyright ownership.
+   #
+   # This program and the accompanying materials are made available under the
+   # terms of the Apache License Version 2.0 which is available at
+   # https://www.apache.org/licenses/LICENSE-2.0
+   #
+   # SPDX-License-Identifier: Apache-2.0
+   # *******************************************************************************
+Module D Documentation
+======================
+
+This is the documentation for Module D.
+
+.. document:: Documentation for Module D
+   :id: doc__module_fixtures_module_d
+   :status: valid
+   :safety: ASIL_B
+   :security: NO
+   :realizes:
+
+
+Overview
+--------
+
+Module D is a base module with no dependencies, shared as a diamond
+dependency by Module B and Module C (both depend on it, neither directly
+via Module A) — used to regression-test that its HTML is merged exactly
+once into Module A's published site.
+Local need link: :need:`doc__module_fixtures_module_d`
+
+Features
+--------
+
+.. needlist::
+   :tags: module_d
+
+Content
+-------
+
+Module D provides foundational functionality shared by Module B and Module C.
@@ -49,6 +49,32 @@ def _sphinx_module_info_fields_test_impl(ctx):
 
 sphinx_module_info_fields_test = analysistest.make(_sphinx_module_info_fields_test_impl)
 
+def _transitive_modules_dedup_test_impl(ctx):
+    """Regression test: a module reachable via multiple dependency paths (a
+    diamond — module_d_lib is a dep of both module_b_lib and module_c_lib,
+    never directly of module_a_lib) must appear exactly once in
+    transitive_modules, not once per path that reaches it.
+    """
+    env = analysistest.begin(ctx)
+    target_under_test = analysistest.target_under_test(env)
+
+    score_info = target_under_test[SphinxModuleInfo]
+    transitive_modules = score_info.transitive_modules.to_list()
+
+    matches = [m for m in transitive_modules if m.name == "module_d_lib"]
+    asserts.equals(
+        env,
+        1,
+        len(matches),
+        "module_d_lib is reachable via two paths (module_b_lib and " +
+        "module_c_lib) but must appear exactly once in transitive_modules, " +
+        "got: %s" % matches,
+    )
+
+    return analysistest.end(env)
+
+transitive_modules_dedup_test = analysistest.make(_transitive_modules_dedup_test_impl)
+
 # ============================================================================
 # SphinxNeedsInfo Provider Tests
 # ============================================================================
@@ -353,6 +379,7 @@ def sphinx_module_providers_test_suite(name):
         tests = [
             # Provider field tests
             ":sphinx_module_info_fields_test",
+            ":transitive_modules_dedup_test",
             ":score_needs_info_fields_test",
             ":score_needs_transitive_collection_test",
 
 
@@ -157,21 +157,28 @@ def test_extra_static_copied_after_main(self) -> None:
 
         self.assertEqual((self.output / "_static" / "logo.svg").read_text(), "custom-logo")
 
-    def test_nested_sibling_module_copy_is_skipped(self) -> None:
-        """Regression test: if a dependency's own HTML tree happens to contain
-        a subdirectory named after a sibling module (e.g. because that
-        dependency was itself built with sphinx_html_merge and already
-        embeds the sibling), that nested copy must be skipped rather than
-        duplicated into the merged output - the sibling is placed once, at
-        the site root, by its own top-level merge entry.
+    def test_deps_are_always_a_flat_transitive_set_no_nested_skip_needed(self) -> None:
+        """sphinx_module.bzl always passes --dep as the transitive closure
+        (SphinxModuleInfo.transitive_modules), each pointing at that module's
+        own_html_dir — a module's own, unmerged Sphinx output, never another
+        module's already-merged tree. There is therefore nothing nested to
+        skip: this merge script has no special-casing for a dep's
+        subdirectory happening to share a name with another dep, and copies
+        it verbatim. Guards against reintroducing the old nested-sibling-skip
+        heuristic, which existed only because deps used to be each other's
+        *merged* trees (recursively containing further-nested deps) before
+        the merge was flattened.
         """
         main = self.root / "main"
         _write(main / "index.html", "<html></html>")
 
         dep_a = self.root / "dep_a"
         _write(dep_a / "index.html", "<html></html>")
-        # dep_a already contains its own nested (stale) copy of dep_b.
-        _write(dep_a / "dep_b" / "index.html", "<html>stale nested copy</html>")
+        # Coincidentally named the same as another --dep entry below. Under
+        # the old nested-merge design this could only happen via actual
+        # nesting and had to be skipped; under the flat design it's just a
+        # same-named subdirectory in dep_a's own tree, copied like any other.
+        _write(dep_a / "dep_b" / "page.html", "<html>dep_a's own dep_b/ subdirectory</html>")
 
         dep_b = self.root / "dep_b"
         _write(dep_b / "index.html", "<html>canonical dep_b</html>")
@@ -182,9 +189,10 @@ def test_nested_sibling_module_copy_is_skipped(self) -> None:
             [("dep_a", dep_a), ("dep_b", dep_b)],
         )
 
-        # The nested copy under dep_a/dep_b/ must not have been copied.
-        self.assertFalse((self.output / "dep_a" / "dep_b").exists())
-        # The canonical dep_b, copied from its own top-level entry, is intact.
+        self.assertEqual(
+            (self.output / "dep_a" / "dep_b" / "page.html").read_text(),
+            "<html>dep_a's own dep_b/ subdirectory</html>",
+        )
         self.assertEqual(
             (self.output / "dep_b" / "index.html").read_text(),
             "<html>canonical dep_b</html>",