Responses
It's actually easy to tell.
Hajoon has this energetic golden retriever vibes, you can also notice his fashion style since it's quites stylish. Hajoon's twin on the other hand has this call and formal energy. His style is also quite formal. You'll know it's the twin if he treats Danwoo with expensive stuffs.
I can't know when they switch up ?when the other twin come ...for exemple in these last chapter did both of them show up ?