fix(extensions): reject duplicate provides.templates/scripts names (#4016)

The resolver returns the first entry matching a declared name, so a
later duplicate within provides.templates or provides.scripts was
silently unreachable while still counted by ExtensionManifest
properties. Reject duplicates at manifest-validation time instead.

Also clarify EXTENSION-DEVELOPMENT-GUIDE.md's provides section: hooks
and events are top-level manifest fields, not provides sub-fields, so
the "at least one of ..." wording doesn't imply they can be nested
under provides.

extensions/EXTENSION-DEVELOPMENT-GUIDE.md

@@ -177,12 +177,17 @@ Compatibility requirements.
 
 What the extension provides.
 
-**Optional sub-fields** (at least one of `commands`, `templates`, `scripts`, `hooks`, or `events` is required):
+**Optional sub-fields:**
 
 - `commands`: Array of command objects
 - `templates`: Array of template objects
 - `scripts`: Array of script objects
 
+`hooks` and `events` are separate top-level manifest fields (siblings of
+`provides`, not nested under it — see [`hooks`](#hooks) below). At least one
+of `provides.commands`, `provides.templates`, `provides.scripts`, `hooks`, or
+`events` is required.
+
 **Command object**:
 
 - `name`: Command name (must match `speckit.{ext-id}.{command}`)

src/specify_cli/extensions/__init__.py

@@ -577,8 +577,13 @@ class ExtensionManifest:
         behavior for extension layers in presets/__init__.py). A present
         'strategy' key is rejected rather than silently ignored, so an author
         who copies a preset-style entry gets a clear error instead of a
-        silently-dropped field.
+        silently-dropped field. Duplicate names within a section are also
+        rejected: the resolver returns the first matching entry by name
+        (``PresetResolver._extension_manifest_declared_template``), so a
+        later duplicate would be silently unreachable while still being
+        exposed by ``ExtensionManifest.templates``/``.scripts``.
         """
+        seen_names: set[str] = set()
         for entry in entries:
             if not isinstance(entry, dict):
                 raise ValidationError(
@@ -597,6 +602,11 @@ class ExtensionManifest:
                     f"Invalid {singular} name '{name}': "
                     "must be lowercase alphanumeric with hyphens only"
                 )
+            if name in seen_names:
+                raise ValidationError(
+                    f"Duplicate {singular} name '{name}' in 'provides.{section}'"
+                )
+            seen_names.add(name)
 
             file_value = entry["file"]
             reason = relative_extension_path_violation(file_value)

tests/test_extensions.py

@@ -1162,6 +1162,29 @@ class TestExtensionManifestTemplatesAndScripts:
         with pytest.raises(ValidationError, match="must be lowercase alphanumeric with hyphens only"):
             ExtensionManifest(manifest_path)
 
+    @pytest.mark.parametrize("section", ["templates", "scripts"])
+    def test_provides_entry_duplicate_name_rejected(self, temp_dir, valid_manifest_data, section):
+        """Two entries in the same section sharing a name are rejected.
+
+        The resolver (PresetResolver._extension_manifest_declared_template)
+        returns the first entry matching a name, so a later duplicate would
+        be silently unreachable while still counted by ExtensionManifest
+        properties -- reject it up front instead.
+        """
+        import yaml
+
+        valid_manifest_data["provides"][section] = [
+            {"name": "dup", "file": f"{section}/a.txt"},
+            {"name": "dup", "file": f"{section}/b.txt"},
+        ]
+
+        manifest_path = temp_dir / "extension.yml"
+        with open(manifest_path, 'w', encoding="utf-8") as f:
+            yaml.dump(valid_manifest_data, f)
+
+        with pytest.raises(ValidationError, match=f"Duplicate .* name 'dup' in 'provides.{section}'"):
+            ExtensionManifest(manifest_path)
+
     @pytest.mark.parametrize("section", ["templates", "scripts"])
     def test_provides_entry_path_traversal_rejected(self, temp_dir, valid_manifest_data, section):
         """The 'file' field is checked with the same path-safety policy as commands."""