So I held up my hand against the black of my t-shirt and it was there too. It's sort of like when you don't focus on something in front of your face and on a point further away so it looks like there's two of the things in front of your face, just not as extreme. So like really slight double vision, but only against a dark background.
So could you please go try this in your nearest mirror and tell me if it happens to you? If so, is this some science thing to do with mirrors I never learned about in high school? And if it doesn't work for you, then which one is dying, my eyes or my brain?