There was a bug addressing this specifically but I can't recall which version it was fixed in and can't seem to find it in the
changes file.
The fix should have repaired your installation - that is, it should have replaced the incorrect error message(s) with the correct one. What was happening, specifically, was the program was showing the "invalid email" error message instead of the "duplicate" error message. I recommend that you check your "Error Messages" page(s) to make sure all of the messages are in the right place.
From what you say you are now upgraded to v1.87d, the latest available - is that correct?
Regards