You would have to go back and delete the duplicate slips. You can click on Income>T-Slips>T-Slip Summary in the left side menu to go through your T-Slips.
However, if you have a lot of slips, it may just be easier to start the return over. To start a new return, select “Start a New Return” from under “My Returns” in the top left corner. Once you have started your new return, you can select “View All Returns” from under “My Accounts & Returns”. Then delete the unwanted return by clicking the small Trash Can next to it or “Delete” under the return’s name.