i personally have never seen two unintentional own goals in one game but i am sure it could happen (wouldn't have to be the scorers fault either if the keeper was dodging around and you pass back to him)
the mod should be able to determine if it was an attempted pass that scored the own goal.. if there was a pass lock when the pass button was pressed, the own goal shouldnt count against you. Other than that, a simple k/b vote mutator should suffice for the rest (maybe i'll make one? :p)