I think the number of examples is roughly half the number of published theorems, so this could be a very long list, indeed. But that won't stop me from making a contribution or two.
The Cauchy-Davenport Theorem. Harold Davenport published it in 1935. Then in 1947 he wrote a paper in which he noted that Cauchy had published the same result in 1813.
Cauchy, A. Recherches sur les nombres, J. Ecole Polytech, Volume 9, 1813, pgs. 99-116.
Davenport, H. On the addition of residue classes, Journal of the London Mathematical Society, Volume 10, 1935, pgs. 30-32.
Davenport, H., A historical note, Journal of the London Mathematical Society, 22, (1947) 100-101.
(References taken from Paul Balister and Jeffrey Paul Wheeler, The Cauchy-Davenport Theorem for Finite Groups, which I found on the web).
