[pmontra]

Those sites probably prompt the attacker with a "We have sent you an email with an activation link" and the owner receives the "you tried to sign up, but you seem to have an address already" message. In this way they don't leak anything to the attacker.

By the way, I've been stuck for years with an ecommerce site that thinks I already registered with them using my email. They're telling me that I must activate the account. I click the link to send me the activation message again but then they say that my email is not registered with them. I'm still stuck and will probably never buy from them because I can't.

[ohbtvz]

I'm confused, you've been trying to buy an item from that website for years and it never occurred to you to just use a different email address? This isn't really believable.

[pmontra]

I just drive to the shop.