Normally, speaking as a game admin who has to deal with double move rules in the Poly BtSDG, it's often alot easier just to allow it and deal with it by being logged in at turn rollover, because if you say no double moves you then have to define a double move, say when it isn't allowed, and generally increase the turn time.
And it's also useless on the first turn of the war, which is the most important, because if the attacker delcares with 5 seconds to go and moves his units then the defender never gets to move his units that turn anyway. Then you get into the arguement that teh attacker should declare at the start of the turn or finihs moving in the first x hours of the turn and it gets real convoluted, real fast.