It's not very unusual, but it's unusual enough. "Dangerous for you", although a big part of it, isn't the only reason for asking you not to come back though. Part of it is that there could be an underlying reason for the faint. It would be bad to exacerbate an undiagnosed or pre-existing condition by repeatedly taking blood. And, whatever reason it is, could (you never know) be blood-borne. The other part is that when you hit the deck, a couple of nurses have to come and attend to you and make sure you're okay. And all the time they're doing that, they're not taking blood from other people. I forget the numbers, I think one nurse is usually monitoring three donors simultaneously? Donation sessions are only a few hours long and they need to be working at maximum efficiency.