If you list the same player in both KR spots, he will play in one of the spots and the other position will be randomly filled by someone else from your roster.

On special teams, you can have 1 player play multiple positions, but it depends on whose on the field at the same time.
The same player can play K and P, but not Gunner. A player playing any of these 3 positions could also play special teams CB or KR, but not both. In any instance, if you save a roster with a player playing in positions that he's unable too, the 2nd position will be replaced by a random player.

Chidash wrote: Special teams does not have a significant impact on the outcome of the game, IMO.

ST doesnt matter unless your teams are evenly matched and get similar rolls on the die on game day

The random element!