"How can it be banned but there is still a party called the KDP?"
Wenn West and East Berlin joined together, one of the stipulations was that the KPD still got to exist. And besides that we also have the DKP which is the KPD but with diffrent colours
"Do you think the KKK and Antifa should be banned?"
Well considering the history behind the KKK, yes. As to Antifa, Depends