After some experimenting, here is a breakdown of the problem I am having. Please can someone check if the same thing happens on their phone? I have asked Google via the Play Music Help site but haven't had a response yet.
1. Clear the queue
2. Go to your albums list
3. tap on an album
4. tap on any song in the album's song list to begin playback
- you are taken to the queue screen where all the songs from the album appear as normal
5. now tap on another song while at this screen OR alternativelty re-arrange a song by dragging it above or below another one
6. now tap on the headphone symbol (top left) to take you back to the albums list
7. now tap on a different album
8. tap on a song in the album's song list to begin playback
- you are taken to the queue screen, however this time all the songs from BOTH albums are listed.
Repeating this process generates a massive list of songs in the queue.
I'm on stock Android 4.2.2, I've tried reinstalling Play Music but it made no difference.