Two players with the same number can't be on the field at the same time. I'd kind of like to be able to duplicate numbers, but you'd have to have formation subs and all that. Also, say you have a DB and WR that are both #5. Well, on kickoffs, the CPU would have to know not to include one or the other on special teams. A prompt would have to come up in the depth chart or the formation sub like a redshirted player would. I don't know if EA could do this, well, they could, don't know if they would. I'd like to have the duplicates but I don't want to see two #5's and 2 #23's on the field at the same time.
The only reasons I'd like to have them is for Recruiting Promises, which needs to be on next gen, promising a recruit a number, that is, and for maybe assigning a number to a freshman I'm about to RS, and a senior has that number.