Well, this site is called Redump because of verifications, actually, not because you personally dump a disc twice or more like it may seem. The idea behind verifications is that it proves a particular hash for a particular version of a game to be error-free, because for two people to get a matching erroneous hash with two different physical objects dumped by two different physical drives is nigh impossible. Of course, this phenomenon is technically possible, and so the more people that have dumped media and reported the exact same hash, the less we fear that matching errors actually occurred because the odds of that happening become astronomically more impossible with each matching hash.
From what I understand about this site's mentality, you should ideally submit a dump for everything you have. Of course, dumps that verify an image can be viewed as the "most important" (along with new dumps perhaps, because somebody has to have something to verify), but it doesn't devalue dumps for media that already have two matching dumps. It won't make the existing dump more "clean", as you put it, if the existing dump is already clean, but it gives people more assurance that if they get the same hash value for a dump they produce from any particular media, then their dump is clean.