At the start of the game, everyone choose one booklet. No duplicates. OK, that's easy.
Let's say that I am playing an Angel, and my friend is playing a Hardholder. When I can change the character type (after the fifth improvement) can I choose a Hardholder, too?
I suppose not. But what if my friend's Hardholder is a Savyhead now? And there are no Hardholder played at the moment?
What if my friend's Hardholder is dead? Or Retired?
What if after a lot of improvement, changes and deaths, there are no unused playbook left? (OK, this is easily solved by using some other playbook found in the web. But let's say that for some reason I can't use them. I have no Internet access, or I am playing in a setting where they are not usable, or I dimply don't like the and I don't want them in my game). The only option left is to create new playbooks (or ask the players to create them)?
No duplicate playbooks is the rule at character creation. It's not the rule thereafter, although most of the time you'll find it most natural to keep not duplicating.
-Vincent