Ooookay. So, I'm kind of at a loss here.
I have narrowed down my issue to some validation rule or something, but I cannot see any rule in place at all. Here's what happens.
Procedurally, I'm trying to duplicate a record on a table. Every time I try, I get a 'duplicate record' error, even though the ID (key field) is unique. This table has many relationships but I don't think that's the cause of the issue. I got fed up, and decided to just manually duplicate the record to see if I get the same error. I tried entering in one field at a time, and it came down to one particular field, let's call it SequenceNumber. Every other field can be duplicated just fine (ID aside) but this field is the one that Access complains about.
So, you'd think I would have found the answer. But no, there doesn't appear to be any validation rules on this field. Or the table. Unless they're invisible?