Activating account email

Hi

I have my site set up so new users need to activate their acount. When they register they are inactive in admin.
When they receive the email and click on the activation link they receive the following message:

Invalid activation code or account already activated!

But when you look in admin their acount is activated.

This will confuse people as they think the link is wrong and they are not aware their account has been activated.

Can anyone help with this?

Thnaks in advance