You didn’t really DISCOVER something if you forget that you discovered it.
I read of a Chinese explorer ca 2200 BC claim that after a long voyage he came to a land (Fusang) that had a spectacular mountain and was covered with forests. A couple of centuries later, an emperor read his report, declared it a fantasy and ordered it destroyed, but a few copies were saved.
Some years ago researchers used his narrative and said his "spectacular mountain" was Mount Hood and his trip could be traced down to California using his landmarks and rate of travel between them. They were intrigued by his description of coming to a "great chasm" in the land when they went inland and speculated that he might have seen the Grand Canyon. A Helluva long haul, but you never know.
If all of this is true, his discovery was not forgotten, just ridiculed, which is even worse than being forgotten. Lots of books on this interesting theory.
Personally, I think others were here from East and West but were either killed off, absorbed by the locals, or as in the Fusang case, not believed and discarded. I would think mitochondrial DNA among the Indians here might prove it conclusively one way or the other.