Custom Error Messages

I am currently using access 97 and would like to know if any one knows of a way to generate custom error messages for Duplicate entries. The records are indexed with no duplicates on a couple of fields, and the data is entered on a form. If a duplicate value is entered the only message that we currently get is that when you attempt to continue it says : “You can’t go to the specified record. You may be at the end of a record set. ”
Any help would be greatly appreciated. Thank you.