cp: issue correct warning and ignore duplicate source dirs

cp: issue correct warning and ignore duplicate source dirs

* src/copy.c (copy_internal): Handle the case where we have the
same destination directory as already encountered, which can only
be due to the corresponding source directory being specified multiple
times.
* tests/cp/duplicate-sources.sh: Add a test for the new multiply
specified directory case, and the existing multiply specified file case.
* tests/local.mk: Reference the new test.
* NEWS: Mention the bug fix.