Peel off as much of the duct tape as possible.
Drench a paper towel with white vinegar. Open the paper towel and press over the duct tape. The moisture allows the paper towel to stay in place on the window surface.
Let the paper towel sit for about 15 minutes. Remove the paper towel and peel off the duct tape.
Place the paper towel back onto the duct tape and let sit for another 15 minutes if the duct tape is still difficult to remove.
Peel off as much of the duct tape as possible. If needed, use a flat razor to get under the duct tape. Carefully move the razor between the window and the duct tape to avoid marring the glass.
Wipe the window with a clean paper towel moistened with vinegar to remove any leftover adhesive and allow to air dry.