Quoting Chris Travers <..hidden..>:
In the db, you can run (in 1.2 or 1.3): ALTER TABLE ar ADD UNIQUE(invnumber);
Thanks, I've just done that but it turns out that I do have aduplicate invoice number. I do have a gap directly below it, though, so I can just change invnumber.
Before I do that, is there anywhere else I will need to change it, apart from ar?
Richard