Multiple Green Lanterns works, but DC has so many to the point now where it doesn't.
In my opinion, (and this is just my opinion, so Green Lantern fans back off), they should only have Hal Jordan, Guy Gardner, and John Stewart, plus a Corps filled with aliens. The decision to create Kyle Rayner was always very questionable to me, and he worked best by himself without the Corps. I would probably turn him into something else and take him out of the Corps. Alan and Jade should exist but in a different reality. The rest of them? I don't see any point to have them, and I actually do think having 34039402948 Earth Lanterns "threatens the brand." It damages it.
The Flash? To the tell you the honest truth, having more than one character on the same Earth with the name "The Flash" makes no sense to me. I said it a while ago, but the Flash's main deal is that he's the Fastest Man Alive. There can only be one fastest man alive. Multiple Flash's doesn't work. Somebody would have to be something else, and somebody would have to be slower, which would instantly diminish them compared to the other. Some character would have to step down in some form.