The type of people that David Duke surrounds himself and support him are both anti-semitic and anti-muslim. Hardly left positions. I'd like to see some examples of leftists engaging in actual anti-semitism....and by that I am not talking about criticizing Israel for the whole Palestine situation. Many of Israel's supporters conflate genuine criticisms with anti-semitism.
There are loads of conservative voices on Twitter that haven't been banned. Probably because they haven't harrased reporters in the halls of Congress.