Well it's very simple and its a bug (but please don't fix, unless you add a repeat function), the only thing you need to do is, to add one song twice to the playlist.
Let's say the song name ist SongXYZ.
1. You add the SongXYZ at the TOP of the playlist.
2. Now add other songs to the playlist, no matter how many - your choice.
3. Put the SongXYZ the SECOND time on the playlist, but this time at the very end of it.
4. You're done - when the playlist comes to its end, it will simply repeat from the beginning, because the SAME song is twice there and it always chooses the first!
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Example, how your playlist could look:
SongXYZ
Song1
Song2
Song3
Song4
Song5
...
SongXYZ
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Screen: