I grew up in the construction business, and have been working in it myself for almost forty years. Trust me - the tape we're referring to, was most definitely used for sealing sections of a/c ducts together for decades.
It's called duct tape for that reason.
I always keep a few rolls of duck tape around. It is useful for a lot of temporary purposes. It will simply not last when used on duct work.
I remember my Father bringing some sort of tape home when he worked at Eglin AFB. I think it actually had a metallic backing and probably would have worked for permanent uses but regular duck tape will not.