It's not "duct tape".  I wrote a blog last night called "Things to do with duck tape and your butt" and a few of the comments were about it being called "duct tape" and not "duck tape". It is in fact "duck tape" It was created in 1938 to navy specifications as a way to patch PT (patrol torpedo) boats while at sea.  PT boats were made of plywood and painted battleship grey. That's where the tape got it's color.  It got the name "duck" from the sailors who used it because it works well underwater. 

Galvanized steel duct work that many people seem to associate the grey tape with wasn't used widespread until the '60s when people started making use of the invention of air conditioning.

The confusion seems to come from hardware stores who market the tape as "duct tape", because that seems to be a good way to sell the stuff. I've seen it sold as duck, and duct tape.  The unused roll in my garage is called "Duck Tape" with a picture of a cartoon duck on the packaging.

Spell it how you want.  We will all know what you are talking about because both versions sound the same.  Just stop being smug jackasses about it.