If my opponent plays a 6 on a territory and marks the territory does that mean that I cannot play a 6 on that territory at a later time? I'm guessing the answer is NO -- each player can use a 6 on each territory once. But a definitive answer would be much appreciated.
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I was going to point out that it is clearly once per player... but looking at the rules, it is not so clear!
However, the wording on the 6 cards agrees with my understanding. They state "you may not play this card there again", not "neither player". Perhaps that is not definitive, but it's close enough for me.