I can only guess at Trump's failure to condemn Duke, tho I believe he had already done it more than once in the past & quickly after that press interview he did repudiate Duke. My guess is that Trump, nearly 70 years old had a moment of non compus mentis, when he could not remember who David Duke was. This could be explained as due to him being exhausted for lack of sleep -- I think he has been sleeping very little, and for having a temporary mental block. Have you ever run into someone you know, but you cannot remember his name? Have you ever gone into a room to do something & forgot why you went there?
It could also have been a "senior moment." Worst of all, it could have been a TIA, a transient ischemic attack, which is like a temporary stroke which passes away. Trump is not young. Of course Trump would not want to admit to that, as it would lead to him being called unfit to be president.
I can't think of any other explanation. The bad ear piece may have contributed, but he did repeat David Duke's name as I recall, & acted like he didn't know about Duke. Jumping on Trump for this or for KKK is really quite unfair.