Is it pronounced duct tape or duck tape?

There's not that much of a difference as far as pronunciation goes. Both of them sound the same unless you enunciate very clearly and separate the Words with a pause, which makes you sound a bit ... precious.

It's properly speaking Duct Tape ... that is, tape for ducts, as in heating and ventilation ducts. However, there's a brand that uses the name "Duck Tape" to take advantage of the common pronunciation.