Do editors ever "overbook" referees?
I think this one will be very hard to answer with hard data. So, I'll just throw in some anecdotal evidence, along with a few things learnt from some editors that I know.

From a reviewer point of view, it has happened to me exactly twice in a decade (i.e. very rarely) to receive an editor's review saying “I've reached a decision based on input from other reviewers, and you do not need to review the manuscript”.


The first time, the email came as my review was already overdue (a week or ten days), so I suppose the editor asked another reviewer when I didn't reply on time, and the new reviewer was fast to reply.
The second time, it was sort of the other way around: I was asked to review a paper, then 5 days later the editor wrote, saying the reviewer who was uncommunicative had finally replied, and my review was no longer needed. He apologized profusely, and offered to actually wait for my review if I had started doing it and wanted to finish. (I was happy to let it go.)


I think for an editor, growing a list of trustworthy and willing reviewers is crucial. So, they simply cannot make them work for nothing! (pun intended)