GA pilot reported multiple NMAC encounters at DRP uncontrolled airport during a training flight.

Narrative

Around XA15Z; as Aircraft X made a descent to the airport from the East and made radio calls; Aircraft Y was spotted North of the airport doing crop-dusting runs around 100-200 AGL. They made a call that said they were North of the airport at 100 AGL. From this point on; no further radio calls were made by Aircraft Y. At XA20Z; Aircraft X made a legal pattern entry for Runway 36 at DRP; crossing mid-field to enter a left downwind. At this point; Aircraft Y was circling South of the approach course for Runway 36; now spraying a farm that was just South of the airport. Aircraft X continued to make relevant radio calls according to each leg. Several; in fact; were directly indicated toward Aircraft Y. When Aircraft X was abeam the touchdown point for Runway 36; Aircraft Y appeared to be on a collision course with Aircraft X. Person A took control and sidestepped to avoid; and Aircraft Y seemed to circle back around; appearing to give way to Aircraft X. Person A radioed Aircraft Y; but no answer was given. Aircraft X continued the traffic pattern into DRP; continuing to monitor Aircraft Y and make relevant radio calls. As Aircraft X turned final; Aircraft Y turned on a direct collision course with Aircraft X at their altitude and did not divert. Aircraft Y was at Aircraft X's altitude within at most; 800 ft. of Aircraft X; and on a direct collision course. (Person B; who was watching Aircraft Y remarked that they could make out what sunglasses the pilot of Aircraft Y was wearing for reference). Peron A immediately took control and executed an evasive maneuver; making a relevant radio call; directly to the pilot of Aircraft Y. Person B; who continued to watch Aircraft Y remarked that as soon as Person A took the airplane; Aircraft Y circled around; although Person B admits that it was unclear whether this was a circle to make another crop-dusting drop or an attempt at an evasive maneuver. Aircraft X went around and as it entered the left downwind; Aircraft Y overflew DRP's runway to the North at an altitude of about 100 AGL with no radio call. As Aircraft X was required to land at that airport as per the flight's unit; it decided to attempt another landing. The landing and taxi back were successful. As Aircraft X turned around on the runway to take off again; Person A noticed on ADS-B that Aircraft Y was once again about to overfly the runway at less than or around 100 ft. AGL. Aircraft X booked it onto a taxiway to clear the way. Once again; Aircraft Y overflew the runway at 100 AGL; again with no radio call; this time from the North to the South. It appeared that Aircraft Y was clear and after Aircraft X repositioned and after a few minutes; Aircraft X lined up on the runway. But suddenly appearing; Aircraft Y overflew the airport to the South yet again; while Aircraft X was lined up on Runway 36 and with no radio call. As soon as Aircraft Y was clear; Aircraft X took off with no delay and exited the pattern with no further incident. Numerous radio calls were sent to Aircraft Y through the airport's CTAF/UNICOM frequency during the entirety of the incident and were not responded to by the crop duster.