Natural language has little to do with logic, and in non-standard, spoken English you find all kinds of oddities.
In this case someone who start saying "it's not..." but decides to substitute the more casual "ain't" could well end up saying "it's ain't". It is common enough that people wanting to emulate a relaxed and free voice in a song might deliberately use this: A lot of the hits on google are for "It's Ain't Easy" or "It's Ain't Over Till You're Crying" and other song titles and lyrics.
Native speakers don't "expand" contractions to understand them. We we hear "It's five o'clock" we don't mentally think "that means "it is five of the clock". Similarly when we hear "It's ain't easy" we don't expand it to "it is am not easy" Which is clearly ungrammatical.