Tests/RunCMake/CXXModules: add a test with duplicate modules

Not in the same executable (as that is IFNDR), but in the same project.
This tests to make sure that there's not some project-wide cache that
gets confused.

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -134,6 +134,7 @@ if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   run_cxx_module_test(generated)
   run_cxx_module_test(public-req-private)
   run_cxx_module_test(deep-chain)
+  run_cxx_module_test(duplicate)
   set(RunCMake_CXXModules_NO_TEST 1)
   run_cxx_module_test(circular)
   unset(RunCMake_CXXModules_NO_TEST)

Tests/RunCMake/CXXModules/examples/duplicate-stderr.txt

@@ -0,0 +1,9 @@
+CMake Warning \(dev\) at CMakeLists.txt:7 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.

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

@@ -0,0 +1,39 @@
+cmake_minimum_required(VERSION 3.24)
+project(cxx_modules_duplicate CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+add_executable(duplicate)
+target_sources(duplicate
+  PRIVATE
+    main.cxx
+  PRIVATE
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        duplicate.cxx)
+target_compile_features(duplicate PRIVATE cxx_std_20)
+target_compile_definitions(duplicate PRIVATE NDUPLICATE=1)
+
+add_executable(duplicate2)
+target_sources(duplicate2
+  PRIVATE
+    main.cxx
+  PRIVATE
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        duplicate.cxx)
+target_compile_features(duplicate2 PRIVATE cxx_std_20)
+target_compile_definitions(duplicate2 PRIVATE NDUPLICATE=2)
+
+add_test(NAME duplicate COMMAND duplicate)
+set_property(TEST duplicate
+  PROPERTY
+    PASS_REGULAR_EXPRESSION "From duplicate #1")
+add_test(NAME duplicate2 COMMAND duplicate2)
+set_property(TEST duplicate2
+  PROPERTY
+    PASS_REGULAR_EXPRESSION "From duplicate #2")

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

@@ -0,0 +1,11 @@
+module;
+
+#include <iostream>
+
+export module duplicate;
+
+export int from_import()
+{
+  std::cerr << "From duplicate #" << NDUPLICATE << std::endl;
+  return 0;
+}

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

@@ -0,0 +1,6 @@
+import duplicate;
+
+int main(int argc, char* argv[])
+{
+  return from_import();
+}