When my brother and I click on the same horse at the same time we both got on the horse but the horse he was riding face the direction I go and when he got off my horse disappear and the horse he was riding kept him on and my horse teleport to him also the moment he got off and the horse multiplied my character got stuck at the middle of the town unable to move. Note we only had one horse. We tried if the bug happen again multiple times and it does. The bug works only in the stable. If player get off the duplicate horse it will be stuck on your avatar while you are not able to ride the original and you can summon it over and over. Duplicate horse rider will be unable to do use items but can interact with surrounding such as crops and doors. When player get off duplicate horse player will still have horse speed.
