Two times click on activation link breakse the link

After a user gets registered a mail is send out to the user with an activation link.
When the user receives the link he maybe clicks on it not only one time or maybe the mail program performs a "precheck" of the URL.
This produces problems for the user workflow, as he isn't able to specify then his password directly in the dialog. The second time he clicks he only gets the information that the activation link is invalid.