Hi Mike and Lars,
Gateway/quiz assignments have the ability to select multiple problems from a pool (see http://webwork.maa.org/wiki/GatewayQuiz_Assignment_Problems and http://webwork.maa.org/wiki/Selecting_Problems_from_Problem_Groups), for which selecting multiple problems from a group will guarantee that you get different problems each time. But there isn't a mechanism to include the same problem twice and guarantee a different seed.
Gavin