Strictly single person use only.
Apart from obvious hygiene reasons, there is a very small chance their fluids could be picked up on another person's scans, especially if they are used concurrently and even if they are cleaned (I have found blood can run to the edges and I have found "sneaky pockets" of blood hiding from me).
Spooky actually recommends single-person single-use for slides, especially blood and body fluid. Use once, discard, because that is a sensible procedure. But I clean them and re-use them. I use alcohol as it cleans well, and kills bugs, and burst cell structures.
I also found the little packets of alcohol wipes are better than a bottle of liquid for cleaning slides because you don't want to get too much liquid on the slides or the edges of the slides as it can ruin them.
I have a filing system of little clear zip-lock bags with a person's name on it. It holds their slides, TENS pads, and extra nail clippings in stickers if I choose to run twin remotes on them. It helps keep them separate and sterile (after being cleaned).
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