They both have the exact same pronunciation and meaning. The only difference is the way they are written. It seems like the US southern dialect to me. Writing "dun" just emphasizes the dialect more than writing "done". The meaning is the same as "I told you", but with more emphasis. But like I said, it's a dialect and not considered proper English. It sounds like a "hickish" dialect spoken by rural people in the countryside.

Both are grammatically incorrect. The only difference between the two is the reduced form of "dun". They mean "I already told you".