I actually used this phrase recently for emphasis. Let me set up the context. I'm finishing up my last semester at college. I've been involved with a particular activity (choir) for seven semesters, and I wanted to continue with it during my eighth, and last, semester, even though I have many, many other obligations. I said to my friend:
"I know I won't have time to do this, but I can't NOT be in choir."
Grammatically, it's a double negative and incorrect, but in this particular context it was the phrase I was looking for. I wanted to emphasis that NOT being in choir is something I am incapable of doing.