Bugfix: Make `backup` respect excludes for duplicate directory entries

Since restic 0.19.0, backing up a directory with duplicate directory
entries always produced "Warning: at least one source file could not be
read", even when those files were excluded. This has now been fixed.

https://github.com/restic/restic/issues/21899
https://github.com/restic/restic/pull/21900
