This man who has such a superior intellect, such great command of current issues and is surrounding himself with the best advisors doesn't know who David Duke is and doesn't have advisors who might be able to clue him in on David Duke? Seriously?
Trump actually DID know. When he was thinking about running for president on the Constitutional Party ticket (I think that was the ticket), he finally bowed owed of consideration with an announcement which specifically referenced David Duke by name, and calling him a racist, and also mentioning that since Duke (and two other folks he mentioned, one of whom was openly a communist) was in the party, Trump did not want to also belong. And then just Friday, two days before his Jake Tapper interview, he responded to questions about David Duke endorsing him and he specifically and by name disavowed Duke's endorsement.
When Tapper asked the question, he said he had not heard of Duke, that he did not even know who Duke was, which is why Tapper was so taken aback.
Trump was doing as Trump so often does, simply playing the media in order to seize control of another news cycle, without paying anything for ads.