He absolutely knows who David Duke is, he's talked about David Duke being a KKK member in the past. He suddenly pretended not to know who David Duke was when he was pandering. He also pretended not to know what the KKK was in that clip, literally everybody knows the KKK.
Here's a quote from Trump in 2000:
“The Reform Party now includes a Klansman, Mr. Duke, a neo-Nazi, Mr. [Patrick] Buchanan, and a communist, Ms. [Lenora] Fulani. This is not company I wish to keep.”
So there is Trump knowing exactly what Duke is, and condemning him. Suddenly 15 years later he doesn't even know what the KKK is, or who David Duke is, and refuses to denounce them?
I don't think Trump is evil, I think he's incredibly sleazy. I don't even think he's a full blown racist, I just think he's just sleazy enough to openly pander to racists.