Often, I hear my locally born friends use "So don't I" to basically me "Me too" or, of course, "So do I". I questioned a friend who used it at work yesterday. I said to him "Why do people here SAY that? "So do-not I" doesn't make any sense! Especially when you mean the EXACT OPPOSITE!" and he could only respond that indeed, it is an odd way of speaking, but that he grew up saying it, as did his parents.
So, what gives?