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I think the contest should differ from regular map creation in some way, especially since no one will forbid participants from using their imagination. There would just be a general description of the map, and the mapmaker can always decorate it in their own way.
Regarding the two maps, it might take longer, but you could have the tournament organizer distribute the maps one at a time, which would result in more diverse maps.
Create a list of who received which map and, after the maps are submitted, hold a vote.