I think, when adding more tracks to an shuffle playlist, the "shuffle session"
restarts. No matter if a track is played twice after that.
Yes, I mean playing to be continued from that position in the already shuffled
list, and the same from the non-shuffle list. Therefor a DB makes sense.
Using a list container would take too long, because of the iterating. Searching
in a DB takes as good as no time.

PS: Does Youtube search work for you?