I think it was one of the worst situations to be in as a DM, that you would have players that had advance knowledge of a module or campaign prior to sitting down and actually rping. Either that, or they would sit down and rp, and then go the store and read the module or a collection of monsters already picked for the campaign.
It kind of cheapens the gaming experience for everyone.
Best thing I think would be, if you have another character, each character be treated as if it had been born to the world, and all is new.