Union All Select
I've got this query in the daily email notification page at Tribe 101:

SELECT Query_PostsWithNewComments.postsid, Query_PostsWithNewComments.sdate, Query_PostsWithNewComments.author, Query_PostsWithNewComments.title, Query_PostsWithNewComments.post, Query_PostsWithNewComments.category, Query_PostsWithNewComments.stime
FROM Query_PostsWithNewComments
UNION ALL SELECT Query_titlelist.postsid, Query_titlelist.sdate, Query_titlelist.author, Query_titlelist.title, Query_titlelist.post, Query_titlelist.category, Query_titlelist.stime
FROM Query_titlelist;

It's returning duplicate posts. I can't remember now why I included the ALL statement, but I know there was a reason. I'm going to remove ALL for the time being and wait to see why it was there.