Validation appears to work but when the record is inserted 'already exists' in database appears even if it does not.

I don't know why there is a solved tick as the issue is unresolved! The validator checks the DB in another web site on the same server with no problem. It was working on this particular web site and then suddenly when it checks to see if a field entry exists it always determines that there is an entry in the db even when there definitely isn't. Any ideas?