BlueBelle, I absolutely understand why you would like to keep your username, having been on here for such a long time. I did change my own username as it was difficult to distinguish myself from another user, and I noticed that it was not applied retrospectively, so all my old posts still had my old username (not really a problem for me as I hadn't been on GN long, though I would have preferred the retrospective change).
Since the PM was emailed to you, it looks as if you will need to contact GNHQ and ask them on your behalf to contact the other user? then a fair thing to do would be for whoever joined first to keep their current name.
By contacting GNHQ hopefully this won't happen again...