Use duplicate class name on an element?

Question

I found that there are many frameworks will check the duplicate class name before adding the new class name on the element that i think will slow down the performance.

Answer 1

There is nothing "wrong" with having a duplicate class name, it's just redundant. There's probably a small performance impact, but that'll only really make much difference if you have a lot of duplication.

Also, preventing duplicates just helps to keep things tidy.