This is a tough one but I think I have finally identified the problem. If you look at the ReturnsPros records (even the ones you had for this project) you will notice that the only thing unique in the rows is the ReturnDate. So if you try to inner join on the six columns between the two tables, you will get duplicates because the ReturnDate cannot be joined since it is null in VehicleDetail. For example, one grouping may have seven return dates, so joining will produce seven times the records that you need to update.

I assume that the partitions are unique but that doesn't help when joining. I haven't come up with a solution yet (the subquery idea hasn't worked so far).

I'll think on it this weekend. Thanks for all your help---and you were right. I HAVE learned a lot from this experience.