Are you using Connect Farm or importing data from a USB into FW Office? If coming from USB, Yes, if you have completed an event and transfer the data onto USB and did not delete the events, they will write to USB a second time and import into as a duplicate job. Most people after confirming those completed jobs that have been imported into FW will select to delete those events from the FmX (Data Files screen) BEFORE recording new Events. You can also remove the current AgGPS folder from USB before transfering from FmX.
Let me know if using Connect Farm. Hope this makes sense.