Leads can be duplicated when used in the off-hand.

1. Send a lead to the off-hand slot.
2. Use the lead in the off-hand on a mob. It will show a red 0 but the lead will remain.
3. Take the lead out of the off-hand, use an empty hotbar slot, and then detach the lead from the mob.
4. Use the glitched lead taken from your off-hand on a mob and detach it. The leads will now be duplicated.