Probably because you have Double Opt-in enabled!
The first thank you page is the page you land on when campaignion is
waiting for the user to verify the email address by clicking on the link.
The second thank you page is after having clicked on the verification link.
If you can't find your Thank you pages via Manage > Content & Actions you can either see them once you click on "Next" and land on the "Confirm" step. You'll have a link to your thank you page there and to your submission page as well. Or simply fill in the form testwise and write down the node number in the URL .
Why Double Opt-in is useful:
On one side, double opt-in is an additional step for your action. Since forms should be as short and easy as possible, double opt in sounds like a step back, but:
It's a decision between list size and list quality. Do you want a big list with a lot of dead adresses or a list that is filled with people who are really interested in your cause?