When doing lm_add(), check for duplicates.

This is useful when lm_find() moves the match to the global mapping,
since lm_find() could be called with a same path more than once.

Reported and tested by: Andreas Longwitz <longwitz@incore.de>
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
MFC after: 1 week