@yorik
I would suspect that there was confusion in the configuration of the differentiated assignments before I would suspect Canvas giving it to someone who it wasn't assigned to.
There's a warning that Canvas gives when you do this kind of assignment:

Someone could misunderstand it and think they needed to get rid of the message. But going back and fixing it is the opposite of what you want in this case. You want to continue.
One thing you can check is to go into your gradebook. If these assignments are right next to each other, you will quickly be able to see if someone is assigned more than one of them.
For example, I had four versions of my final challenge. Before the challenge was taken, they had a dash for their grade.

If students were assigned more than one quiz, then there would be multiple entries in the same row. You want for there to only be one assignment for each student.
If you find multiple items, then that gives you a place to go and try to track down the issue.
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