It is possible that two $25,000 checks are placed simultaneously in different boxes. This changes the odds of picking the check but not by much.
I believe that is the most plausible theory.
Say if box 7 and box 8 both have a $25K check in them and no one picks them: I don't think Standards and Practices don't care if there is more than one box with the grand prize just as long as they reveal and show *either* box 7 or box 8 with the check at the end to prove there was *one* check in *one* box worth $25k. If in the first game the contestant chooses box 7, one check is revealed (win or lose), and thus box 8 will either have to be selected by the round 2 contestant, or that number and box will be revealed by Geoff and Emil and Geoff at the show's end.
I think the bigger concern is that if there were multiple boxes with the 52-day cruise at the start and the first was chosen in round one, they would need to replace the other box(es) that also had the cruise with a back-up prize or klunk. It would look more awkward if both the round one and round two chosen boxes were the same prize or klunk than if there were two $25,000 checks.
So by the end of the show, you either have one check in a box to reveal, or you've already shown that there was a $25k check available in both rounds and both contestants picked the "grand prize". Whether the contestant kept the box or took the initial cash is a moot point.