To be really technical about it: there's a reasonably popular brand of tape called "Duck" (https://www.duckbrand.com/). They refer to their main product, which is a kind of duct tape, as "Duck Tape." To clarify that the term "Duck Tape" refers only to their company's product, they refer to it as "Duck Tape® Brand Duct Tape."

So the phrase "Duck tape" is fine, if you capitalize the first word and use it to refer to tape made by the Duck brand. Likewise, "Duck Tape" is fine, if you capitalize both words, since it is the name of the specific product line made by Duck.

In fact, if you call store-brand adhesive bandages "Band-Aids," and you call store-brand acetaminophen "Tylenol," you could argue that it makes sense to call store-brand duct tape "Duck Tape." And if you don't always capitalize "Band-Aids," you could argue that it makes sense not to capitalize "Duck Tape" either.

So I would argue that using the phrase "duck tape" to refer to duct tape is fine, provided you accept that it is effectively a genericized trademark.