Merge topic 'duplicate-source-behavior' into release-3.31

3e15419bd4 target_sources: Restore toleration of duplicate CXX_MODULES sources
5cfb8ae790 Tests/CXXModules: add a test with duplicate sources

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !10155