Here's a simpler way to get a table of records where Part ID is unique in every record.
Open field options for the Part ID field in the Parts table. Click the validation tab and specify "unique values" and "validate always". Now import the same data you've been importing into Tests into the Parts table mapping only those fields, such as Part ID that contain data you need in this table.
When you import, the records with duplicate Part IDs will be automatically excluded from the table.
I also recommend that you add a new field to use as the Primary Key field in place of the externally provided Part ID. You can load this field with serial numbers and set up the field to auto-enter serial numbers for new records created. Then you can change over to using this field instead of Part ID in your relationships.