Can we unknow what we already know?

I'd say yes.

One way would be if (for whatever reason) you ceased to believe some proposition P that you formerly knew to be true. If belief is a precondition for knowledge, then you'd no longer know that P.

Another way might be, while retaining your belief of P, to come to believe (or indeed even know) some proposition Q your belief of which undermines your justification for believing P and thereby deprives you of the knowledge of P that you formerly had. See Carl Ginet's article "Knowing Less By Knowing More" (linked here).

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