In a patent, what matters are the claims at the end. These are what defines what the inventor claims to have a right to exclude others from doing.

If a patent claim covers X, Y, or Z and it turns out that yes, X was known in the art, then yes, the patent will be invalid.

In plain English: Suppose someone claims to have invented doing a thing once, twice, or three times. That patent would give them the right to exclude anyone from doing that thing once, twice, or three times. But if people have been doing that thing once for years, the patent owner is attempting to exclude people from doing things that people already do. That's invalid. The court will not rewrite the claim to save it to being only 2 or 3 times--there may be other ways to do it, but the court is not that way.