3 files changed, 26 insertions, 0 deletions
diff --git a/Tests/RunCMake/project/LanguagesDuplicate-check.cmake b/Tests/RunCMake/project/LanguagesDuplicate-check.cmake
new file mode 100644
index 0000000..31c7da3
--- /dev/null
+++ b/Tests/RunCMake/project/LanguagesDuplicate-check.cmake
@@ -0,0 +1,10 @@
+if(NOT actual_stderr MATCHES "The following language has been specified multiple times: C\n")
+  set(RunCMake_TEST_FAILED "'LANGUAGES C C C' should report only 'C' and only once.")
+endif()
+
+if(NOT actual_stderr MATCHES "The following languages have been specified multiple times: C, CXX\n")
+  if(RunCMake_TEST_FAILED)
+    string(APPEND RunCMake_TEST_FAILED "\n")
+  endif()
+  string(APPEND RunCMake_TEST_FAILED "'LANGUAGES C C CXX CXX' should report 'C' and 'CXX'.")
+endif()
diff --git a/Tests/RunCMake/project/LanguagesDuplicate.cmake b/Tests/RunCMake/project/LanguagesDuplicate.cmake
new file mode 100644
index 0000000..97a79d0
--- /dev/null
+++ b/Tests/RunCMake/project/LanguagesDuplicate.cmake
@@ -0,0 +1,11 @@
+cmake_policy(SET CMP0057 NEW)
+
+project(ProjectA C C C)
+project(ProjectB C C CXX CXX)
+
+get_property(langs GLOBAL PROPERTY ENABLED_LANGUAGES)
+foreach(lang C CXX)
+  if(NOT lang IN_LIST langs)
+    message(FATAL_ERROR "Expected language '${lang}' to be enabled.")
+  endif()
+endforeach()
diff --git a/Tests/RunCMake/project/RunCMakeTest.cmake b/Tests/RunCMake/project/RunCMakeTest.cmake
index 945d9ed..6d9f52f 100644
--- a/Tests/RunCMake/project/RunCMakeTest.cmake
+++ b/Tests/RunCMake/project/RunCMakeTest.cmake
@@ -12,6 +12,11 @@ run_cmake_with_options(CodeInjection
 if(CMake_TEST_RESOURCES)
   run_cmake(ExplicitRC)
 endif()
+
+set(RunCMake_DEFAULT_stderr .)
+run_cmake(LanguagesDuplicate)
+unset(RunCMake_DEFAULT_stderr)
+
 run_cmake(LanguagesImplicit)
 run_cmake(LanguagesEmpty)
 run_cmake(LanguagesNONE)