Not an optical illusion.
I kept looking at it too to figure out what it was, and even with a 9x magnifier, best I came up with is that it's maybe a scratch in the copper with the zinc exposed.
I don't think it's doubling at all, then again I've never actually seen doubling in hand, so I can't say with 100% certainty.
Anyway, here's about as close as I can get with my camera:

I kept looking at it too to figure out what it was, and even with a 9x magnifier, best I came up with is that it's maybe a scratch in the copper with the zinc exposed.
I don't think it's doubling at all, then again I've never actually seen doubling in hand, so I can't say with 100% certainty.
Anyway, here's about as close as I can get with my camera:

























