This might sound bad and greedy coming from me since it is projected I will be the number 1 contributor this month, but oh well. I think that eliminating rewards for those in the top ten is not a good idea. There are other folding teams that give out rewards as well and if the in game rewards are eliminated, people might switch teams. I know that I would.
With that being said, if you want to redistribute points from repeat top 10 winners, I think it would make more sense to only provide 500 merits to the top three winners and distribute the rest to others after winning 1 or 2 months in a row.
I agree that having the 20 random winners not repeat. To keep the incentive, I would think a scale making it more likely for people to get the random rewards if they haven't gotten it and less likely if they have gotten it or gotten it recently.