the kehillas yaakov in his last siman on kiddushin discusses this situation
if only one of a person’s numerous anccestors were mazane they are mazerim
If there was one mamzer about a millenium ago then we all mamzeirim
Nonetheless we are not choishesh
and the last mishna in eidiyois says that eliyohu hanovi will not be meracheik the unknown
R’ Menashe Klein also dealt extensively with this topic and basically says that Hashme will never allow it to happen