I think that Newton's and Leibnitz's coming up with calculus independently and at almost the same time is pretty strong proof that these mathematical theories are discovered.
Actually, several scientists coming up with approximately the same thing at approximately the same time is not that surprising to me. Evidently they "network" among themselves quite a bit and will occasionally approach a challenging question (like von Neumann's) somewhat competitively.
Von Nuemann asked the "most intriguing, exciting, and important question of why the molecules . . . are the sort of things they are." The Physics of Symbols - Pattee
Like two patent applications for the telephone being filed on the same day. Was the telephone discovered?