For example, "I can't spar with my dead brother, not unless he climbs out of his grave."
I looked up the term "not unless" in Merriam-Webster site and it's an idiom that means "only if".
So from my understanding, I think the above sentence means, "I can spar with my dead brother only if he climbs out of his grave."
What if I remove the not preceding unless?
"I can't spar with my dead brother, unless he climbs out of his grave."
So if the dead brother climbs out of his grave, he can spar with him.
What exactly is the difference between "unless" and "not unless" then? They both seem the same to me, at least based on this example.
Best Answer
As you say, in this case, the presence or absence of not doesn't change the meaning.
I can't spar with my dead brother unless he climbs out of his grave gives the meaning, but sounds a bit flat, as though his leaving his grave were a likely occurrence. (Compare You can't travel by train unless you buy a ticket.)
I can't spar with my dead brother - not unless he climbs out of his grave! makes the second half of the sentence an afterthought, emphasising that the sparring would only be possible in the event of a supernatural occurrence.