Revert "(#11423) Clearer error message about duplicate imported resources."

This reverts commit e8e1f577ba091203b05b4a8279aadd8c55a65fe7.

This was committed directly rather than as a pull request because I misread
the GitHub "online code editor" button, and it didn't behave the way I
expected.

I intended a pull request, rather than a commit, and will reissue this the
right way. Sorry. :(