I don't get how that is a double negative.. Surely if it were it would be 'ain't you never seen'? Or such like. As to you saying no. I understand perfectly well you mean 'no I have seen '
Ah good. If I'd have just said no though,would you have thought I hadn't seen it? It's wierd one because it's the same with haven't, which is the abbreviation of what you said, and most people would answer yes to 'haven't you seen the Holy Grail?' Anyway, yes I have seen it but certainly haven't memorised it so no idea what you're on about. Swans though, they can break your arm with their wings.
To be a true double negative it would be havent you not seen it? So had you not have incorrectly inferred it was a double negative I would rightly have assumed you had not in fact seen said film.
The double negative comes from me answering no to your question to tell you I have seen the film. Have you not seen the film? No. Meaning yes I have. No plus no = yes.
Putting my pedantic hat on (and no, pedants can't go in Room 101) that's actually not a double negative, it's an intensifier to a negative. The full response would be "No, I have not seen the Holy Grail" which is in no way a double negative. No they can't (again, not a double negative ). Not unless you have osteoporosis and hold your arm at just the right angle.