Proper place for validation

[Janne Mattila]

I am wondering what would be the propert place for some validation code. 
Let's say I have a registration page where user inputs his/her information 
for a new user account. There's a field for e-mail address. On one level 
that information is validated on the ActionForm, for things such as

- field is not left empty, if it is required
- field is a valid e-mail address
That much seems quite clear. But where should I put a check that ensures 
that same e-mail address is not already used by an existing account?

On the other hand, I could put it in the registration action, because 
checking that is (kind of) user-input validation, and action is a natural 
place to do this.

On the other hand, I could put it in my business class. The pros for this 
would be smaller and simpler action code, and reuse if same check is needed 
with different actions.

Any suggestions? Intuitively, I think I would put that check in the Action, 
but I am happy to hear any arguments in favor or opposing that.

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