You must clear your cookies. This should be done after every offer you complete. If you are not sure if you have deleted your cookies, then delete them before doing an offer.
For Firefox I suggest Selective Cookie Delete.
This is an excellent addon for those of you using gpt sites & doing offers. The addon places a little cookie in the lower right corner of your browser.
When you click the cookie a box will open & you add the sites you do not want to delete cookies from.
You can edit your list by clicking EDIT SITE LIST.