Some folks on certain medications (or just because) can have an increase in red blood cell production, sometimes called "blood thickening," I believe, and they can sometimes just get a prescription from a doctor to go to a donation facility. Or some facilities will still just allow you to do this if you tell them you need to get rid of excess blood for the reasons above, and then they dispose of it. (Not everyone requires a doctor's prescription for it.)

That's just one use case, obviously, so there is a system for some to get rid of excess blood, at least, in a similar manner as you would donate.