We have often seen theorems proven multiple times in our field, because the result was proven on both sides of the iron curtain (e.g. Cook and Levin), sometimes it is just easier to prove a lemma then work through the literature, or we just simply didn't realize someone else had thought about the same problem. We have a considerable number of published work in our field and you cannot hope to know every "known" result, even in an area where you are considered an expert.
There is no ethical breech if you reprove someone else's theorem as long as you make good once you learn the result already existed. Although I have occasionally reproven theorems I had seen previously in talks or papers I've reviewed and that's just downright embarrassing.