Hi,
string Usercommand = "INSERT INTO People_Event (People_Event.Confirmation, People_Event.People_ID, People_Event.Event_ID) VALUES ('no', '" + peopleID + "', '" + DropboxEventID + "')";
This statement does not move people from 1 event to another - an INSERT statement will add records, not update existing records.
Have you checked the People_Event table directly? ... rather than viewing the data via the front end application?