While playing on my friends private Minecraft server, I was riding a horse around. Later, while still on my horse, I logged off. When I got back on the server, I noticed there was another horse inside of the horse I was on. The horse had been duplicated. I noticed this and quickly realized what had just happened. I kept leaving and joining, and I was 100% sure that horses were duplicating everytime I left while on my horse and joined back. This horse duplication trick only works with the original horse.