I also have the same issue in my database
I get the same warning
More than one INDEX key was created for column comment_approved
I do have two ROWS in the same COLUMN that start out with comment_approved
(see below)
comment_approved
and
comment_approved_date_gmt
I am going to do an experiment and remove the later and see if that works. Perhaps will loose date stamp with my comments, but I can live with that.
I really hope this does not mess everything up!
-Fred,