Is there a reason why not? I guess they just wanted him both as a fighter and a support in the game, not much more.
As for what happens when you choose Croagunk as a fighter and a support, nothing happens. Support Pokemon can only spawn when the support gauge is full, and leaves almost immediately after they appear, so its not likely that you will confuse which one's your Fighter and which one's your support, especially since your fighter is focused on the camera. If 2 fighters are the same Pokemon on the other hand, one of them will change color.