The Import function is only additive - it does not check for or update existing records. The best way to address this would be to delete the old records prior to importing the new, aggregated records from PS. Alternately, if you are able to get incremental records from PS (not pre-aggregated), you could allow the DA to peform the aggregation through views, custom fields, and reports.
Another option might be to create a web services integration that would allow you to update existing records rather than deleting/inserting records each month. This would take more work up front to develop the web services piece, but would eliminate the ongoing administration of the DA data through Daptiv.