I only checked a few so far. They also have some that allow you one prize per level. It's really at the sponsor's discretion to decide whether or not to limit the prizes. ePrize is just the messenger.
I asked this because I won a Mypolex bar, checked the rules but didn't see anything disallowing more wins. This was confusing because in the past they had the rule for so many contests that you couldn't win an instant prize twice.
I kept playing that one after I won, because I didn't see any limitation in the rules. It does also say that for each day you play the instant, you get an entry in for the grand prize.