Tests/CXXModules: add a test with duplicate sources

See: #26549

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -321,6 +321,7 @@ endif ()
 
 # Tests which require collation work.
 if ("collation" IN_LIST CMake_TEST_MODULE_COMPILATION)
+  run_cxx_module_test(duplicate-sources)
   run_cxx_module_test(public-req-private)
   set(RunCMake_CXXModules_NO_TEST 1)
   run_cxx_module_test(req-private-other-target)

Tests/RunCMake/CXXModules/examples/duplicate-sources/CMakeLists.txt

@@ -0,0 +1,19 @@
+cmake_minimum_required(VERSION 3.31)
+project(cxx_modules_duplicate_sources CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+add_executable(duplicate_sources)
+target_sources(duplicate_sources
+  PRIVATE
+    main.cxx
+  PRIVATE
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        duplicate.cxx
+        duplicate.cxx)
+target_compile_features(duplicate_sources PRIVATE cxx_std_20)
+
+add_test(NAME duplicate_sources COMMAND duplicate_sources)

Tests/RunCMake/CXXModules/examples/duplicate-sources/duplicate.cxx

@@ -0,0 +1,8 @@
+module;
+
+export module duplicate;
+
+export int from_import()
+{
+  return 0;
+}

Tests/RunCMake/CXXModules/examples/duplicate-sources/main.cxx

@@ -0,0 +1,6 @@
+import duplicate;
+
+int main()
+{
+  return from_import();
+}