Find the serial number field in Manage | database | Fields and double click it to open field options.
On the validation tab, specify "unique values" and "validate always".
Now, during import, duplicate serial number records will be omitted from the import.
Fantastic! Thanks... I figured it was something simple I was missing. Will that also automatically create a serial number if you import a record where that field is blank??
It will not. If you enable auto-enter options (check box in very last dialog to pop up before the import starts) for the import, this enables ALL auto-enter options and your imported records will all get new serial numbers.
What you can do, is perform a find for records with an empty field immediately after the import, then use Replace Field Contents to assign serial numbers to these records and also update entry options at the same time.
Ah yes... makes sense. Thanks again Phil. :)