I believe if you have a field set up to validate as unique, then this would disallow the importing of duplicate records. Typically this would be the PK which should be auto entered serial and set to validate as unique always.
That is true, but with my issue, the incoming data does not have the pk number assigned to it. That happens when the record comes in. It does, however, have a unique serial number. What you mentioned would work, but I have other records that have a blank in the serial number field. In order for the table to interact with other processes I have set up, the fields in those records without serial numbers have to stay blank.
Because a blank field in not unique, that solution won't work. I did try that, had problems, then I realized the blanks are the problem.
I don't see why the blanks would be a problem. I just created 5 new, totally blank records in test file where the single defined field is set to only allow unique values.
Or do you have problems during import when this field is blank?