Queuing a track in iOS, which is located later on the playlist which is currently playing will cause the track to play twice. This happens due to all of the tracks between the hard queued copy of the track, and the soft queued (playlist) copy of the track being skipped out of the play queue.
This appears to have been introduced as a bug in the desktop client in v0.9.10.x as well.
Play a playlist
Queue any track which is later on that playlist.
Once the track plays once, the Spotify app will skip all tracks between the two queued copies of the track in the playlist, therefore playing the track twice.