I was trying to take my horse to the nether. I couldn't push it in my portal however, because of the height difference, so I decided to use a lead.

When my horse went to the nether, I followed him. There was a strange thing about the lead: it had dropped, because the connection was broken, but I still held another one in my hand when I got there.

I know there have been several bugs in the past which made it possible to "copy" items. I think this might be another bug of that kind.