David Duke is a red Herring.Because of historical circumstances, there are not that many Republicans known to be former members of the KKK. However, David Duke (former Grand Wizard of the KKK and current neo-Nazi) has found more acceptance on the Republican side of the aisle than with Democrats. Yet. the Democrats I know accept the word of Republican national leadership that disavows Duke as not representing their values.
I don't see the double standard you claim.
He only won one election. It was in a unique district, to say the least. He was never accepted by The GOP or it's leaders. Byrd was a leader of the Democratic party on a national level and was fully embraced by the party. The Democrats gave Trump literal hell because he did not denounce Duke. Trump said he was not familiar enough with Duke to form an opinion on him. He later denounced him. I had the same opinion of Duke at the time the question was asked. I had no idea the man was alive or what he believed then. He could have been the NAACP chairman by then for all I knew. Trump thinks like I do. If someone asks you a question that seems to have an obvious answer, don't answer it until you research it. It's usually a trap. That's how he saw the Duke question.