If you enable this option at the very bottom of your config by setting it to true:

You will only be able to win the same reward once. So lets say you get a monthly sword, you will not be able to win that monthly sword again because you cannot get the same reward (in this case the monthly sword) twice.

I would recommend you have loads of rewards (like 20 plus) if you want this option so that it makes sense to your players, but if you only decide to have like four items for example it will win all of them and then go back to as if this option was false and it would give completely random items again.