If the rules allow it, then so be it.
It seems gauche, but unless the rules specifically state that you can't re-enter the same costume in both hall and masquerade contests, then there's not much you can do. If the rules changed to allow it, there must be a reason. If you think it's tacky or unfair, express your displeasure to the con staff who made that rule.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com