This happens in layouts like "Playlist, Library & Context": when double-clicking a song in SongList with the option activate=addplay, it is added to playlist twice. It's been like this for at least several weeks now, presumably since this (http://gmusicbrowser.git.sourceforge.net/git/gitweb.cgi?p=gmusicbrowser/gmusicbrowser;a=commit;h=10c7b87189f6dbe9f2a77370676362f4526f79ae) update, though I'm not sure. Admittedly it's not that much of a problem since alternatives (enter, spacebar, drag-and-drop, etc) work normally.
thanks for reporting it, I fixed it.