It started when I was leading two horses back to my house while riding on a donkey. I was moving backwards so I could make sure they made it between a couple trees. Then one of the horses suddenly disappeared along with the saddle and lead attached to it.

I decided just to lead the single horse back for the time being and had planned to report his disappearance as a bug, but I figured I would check the area where the horse was to see if he reappeared. I did find him, but I also found a duplicate horse. I closed Minecraft to see if anyone reported this sort of bug and when I reopened it there was another. Every time I opened the world it made another duplicate. I included a couple snapshots of the event. You can find my build info in the first.

I do run Optifine, but because of the nature of the mod, I doubt it was responsible for the bug.

Edit: I've confirmed that the duplicated items are usable.
The source of all this seems to be some bugs with leads. I found him about at the bottom of the hill that he disappeared on. I'm guessing that although I saw him at the top, he was actually stuck at the bottom of the hill and when the lead finally broke, he disappeared because he was never actually there. I don't know how this causes the duping.