I think it is just a genetic oddity, like babies born with extra toes or fingers...I had a cabbage do this...in a row of 5-6 plants only one of them is growing leaves from it's leaves...it just looks like a flap though, cuz well, it's a cabbage...
I have had males make pollen sacks from the leaf base in little clusters, but never seen a leaf in a leaf...You should press that in a big book and keep it...