Update, in case anyone cares:
I found a way around the above error- the cause is the students who are carried over from the previous course (in my case there were 3 such accounts: knightly, admin, global_user). You have to unassign all the hw from those users (preferably at the beginning of the semester) and then reassign them. If you do this, the error goes away.
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Thomas Hagedorn - Copying a Course -- Course Administration Page - Webwork 2.1
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