Hi David,
Since you did Full load first and then changed the DTP to delta, it loaded entire data again and you got duplicate records. This is because delta doesnt get set in full load DTP.
What you can do is -
1) Delete recent data request (delta dtp request) from cube and manually run the DTP once with 'No data transfer; delta status in source, fetched' as processing mode in Execute tab.
Now next time onwards you will get only delta records in cube.
OR
2) Delete entire cube data and simple run Delta DTP everytime. For the first time it will fetch all data from PSA and next time onwards only delta records.
Hope it helps.
Regards,
Prathamesh.