Are you listening continuously, or do you ocassionally quit Pocket Tunes?  When you re-enter, the list will be re-shuffled, and you may hear the same song again.

Also, if you have the repeat option enabled, then you may get several songs playing near each other when the playlist starts over again.  (It is re-shuffled each time it goes through the playlist, and you may randomly get the same song to repeat soon after it was played.)