Enderdragon Duplication Bug

I was derping around on my computer with a friend on LAN and when I got off and came back on a second enderdragon appeared.

What I expected to happen was...:
I thought I was just going to derp around and play some of the new features. So, I thought I was going to kill the Enderdragon and get back to the overworld.

What actually happened was...:
I got on the world and I saw a a full health bar on the Enderdragon and then later noticed a second Enderdragon

Steps to Reproduce:
1. I believe go into The End with a friend on LAN.
2. We were using anvils and horses and I tamed a horse put a saddle and enchanted diamond horse armor on by using the anvil in The End.
3. I and the friend were killing the Enderdragon and brought its health down to half.
4. I got off because I had to go and I noticed the health was back up to full after bringing it down to half when I saw another Enderdragon when I got back on.