Ignoring one of the emails that are sent if group_mentioned and user_watching_first_post

Hi everyone,
I can’t easily test it on meta (or “try”) but I have tweaked only a very limited set of email parameters on my instance ( 2.9.0.beta2 ff530264f4 ).

An email is sent twice when a user that is watching the first post of a category is also member of a group that is mentioned (in the first post of a topic of said category).
One of them should probably be skipped.