If they didn't take David Duke seriously, why was he at the rally and why is he a spokesperson for the rally after the fact?

You seem confused about the fact that this rally was organized by white supremacist, fascist, and Nazi groups including the KKK, formerly headed by David Duke.

And by the way, the only pictures I've seen of protestors with shields and helmets and weapons are pictures of the white supremacist/fascist mob.  Not to mention marching with torches.  If the anti-fascists were wearing helmets, I wouldn't blame them.