You shouldn't have sealed the contest because some of the contestant will just copy/paste someone's logo in other sites or google it and then submit it. If you think that the other contestants is just ripping off other fellow contestants design you should reject their work. You'll just waste money by choosing the copy/paste design. Its better if you unseal or not seal the contest so that other contestant the other contestants designs so that they can complain.
If you choose the rip off design or the copy/paste design from another design website. Then the design website or the owner of that logo will sue you for copyright infringement.