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Yeah, I'm with Mike - these are prone to cause you some real trouble. Given how many there probably are and how often they change/rotate, I'd strongly suggest using rel=canonical or some not indexing the alternate offers somehow. They may be necessary for users, but these pages aren't all necessary to have in your index. By trying to rank for every single one, you risk harming your more important rankings. Honestly, as Mike said, Google can't even really tell these are different, except for the URLs, so even the long-tail ranking benefits are nearly zero, I suspect.