preventing sign up forms from being used for user enumeration

Hi
I'm developing a application which requires users to sign up with both
a username and an email address. I only want an email address to sign
up once and don't want duplication of usernames.

If I just put up a warning stating that an email address is already
registered if it is, the form is open to being used for user
enumeration. Apart from using things like captchas to try to defeat
automated attacks, is there any way to stop this?

I know on things like forgotten password forms you can ask for extra
info so someone guessing would have to get both bits right but I can't
think of a way to do this here.

Robin

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