Business are business wouldn't make sense. We could say "businesses are", but if it's just business you need to use "is". The phrase "X is X" is typically used to reinforce or remind someone of one of the qualities of X. In a show like Breaking Bad, they are probably using it to remind whoever is present that business is cold and emotionless. I'm guessing someone died in this scene? They want to remind whoever else is there that the death is just part of business. You'll hear this with a lot of things, including names. For example: someone might say "John is John" (or often "John will be John") if I do something that is quite typically of me (like tripping and falling -- I am very clumsy haha). They are expressing that this quality or action is something that is not a surprise. Instead, it's quite typical.

Hey Daniel, As a matter of fact, the correct way to say it is ''Business is business'' - this expression is considered to be a way of saying that financial and commercial matters are the important things to consider and you should not be influenced by friendship, etc. E.g.: ''He’s a nice guy but business is business. He owes us money and he’ll have to pay it back.'' ''Business is business'' is a phrase that emphasizes business decisions as completely separate from emotions or personal issues. E.g.: ''You're only being traded because the team needs to dump some salaries. It's nothing personal—business is business.'' Hope that helps!!!

According to the MacMillan dictionary, the phrase is correct.