How to reproduce:

1. Put two or more tnt minecarts down on an activator rail, possibly using a dispenser.
2. Place armor stand next to rail and put a single skull and some armor on it on it
3. Activate the activator rail, let the explosion happen and pick up loot: You will have multiple skulls and a few ghost items on the ground.

Originally found here: http://www.reddit.com/r/Minecraft/comments/2eoegm/head_duplication_in_minecraft_18_pre_2/