Turns out plain dictionary requires keys to be unique.
So we need to store values as lists.

This way there are no more false positives for <set> as in
include/sfx2/linkmgr.hxx and include/sfx2/objsh.hxx

Thanks Miklos for the advice!

Change-Id: Ie2cfb63644d6cbd51171eb95e3bcdd9246343efa
Reviewed-on: https://gerrit.libreoffice.org/69953
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>