Songs playing simultaneously in MusicBee has been reported before, and has been some sort of enigma for a long time.
I have also experienced it (a couple of months ago, and if I recall correctly it was two different songs playing at the same time), but I couldn't replicate it in a way that allowed me to create a useful bug report.
Maybe it needs a couple more drinks to replicate. (I'll start with that now)
So all that I can contribute to this is to confirm that simultaneous playback is indeed occurring sometimes.
And that I have no clue whatsoever about the circumstances when this is happening.
If you are able to replicate this in a reliable manner, it might help if you could provide some useful information such as MB version, audio settings, audio devices, etc.