Duck Tape is now a brand name of adhesive, plastic-coated, fabric tape, but it was originally the generic name of strips of duck material. The original was not adhesive, but was useful in applications from shoemaking to preventing the corrosion of steel. Neither the original nor the current is suitable for using on heating ductwork.
Duct tape is now the more common, generic name of adhesive, plastic-coated, fabric tape. It also is not suitable for use on heating ductwork.
If you need tape to seal your heating ducts, be sure that it specifies that it is made for that purpose. It will be more like foil than cloth tape.
If you are interested in making clothing from duct tape, as mentioned in last week’s Muddle, and you are a senior in high school in the United States or Canada (with some regional restrictions), check out the Duck brand contest. Each year they provide scholarships to the creators of the best prom dress and best tux. If you are interested in knowing more about it or seeing winners from previous years, you can do that here: https://www.duckbrand.com/stuck-at-prom
(I am in no way affiliated with Duck Tape.)