When pushing a horse through the nether portal the lead pops off. When picked up, there's 2 leads. Can also lead the horse through by running around to the other side of the nether portal and having the horse walk toward you. When the lead pops off, there's 2 leads on the ground. The lead will could end up in the overworld or the nether.

This looks like it is duplicates mc-15270 that was fixed in 1.6