What about them? The basis of those synthesized (and modified) mABs is still convalescent patient serum, from which those (unmodified) antibodies have been extracted and studied.

Somatic recombination in b-cells and t-cells is essentially a numbers game, leading to a variety of b-cell receptors and consequently antibodies. When one of those b-cells recognizes an epitope which belongs to a pathogen, it's pure coincidence, and only after that the clonal expansion and antibody production of this specific cell is initiated.

That's why the search for neutralizing antibodies for SARS-COV-2 still continues, because new types of neutralizing antibodies are still found in patient sera, which might be more potent in preventing severe disease.

To answer briefly: The chance that two people produce the same antibody (ies) is non-zero but rather low. How effective in neutralizing an antibody is depends on a lot of things, pure chance being the most important.

Prions are not microorganisms. They are proteins, which can misfold and thereby lead to prion-diseases.

Therein lies the problem of curing these diseases. There is no easy way of destroying proteins in the body, much less so if they are not inherently foreign to the immune system.

This might be of interest: https://www.thelancet.com/article/S1474-4422(22)00082-5/fulltext