Fix duplication of dropped items on mob death.

 * The EntityDeathEvent handler contains a work-around for a vanilla Minecraft
   bug wherein mob equipment with a drop chance of 100% does not always drop.
 * Prior to Bukkit commit d611cff, the EntityDeathEvent did not include mob
   equipment in the list of drops, so it was safe for Doppelganger to simply
   strip the mob of its gear and add those items to the drops.
 * In newer versions of Bukkit, the equipment is present in the drops and is
   also still on the mob itself, so the Doppelganger work-around code leads
   to duplicate items in the drops.  This commit avoids adding equipment to the
   drops when an identical item is already present in the dropped item list.
 * This fix works for older Bukkit versions as well.
 * As described at https://github.com/Bukkit/CraftBukkit/commit/fb58cc92375d85bfe7a3dc8671c70d1a4e308e3c
   the new Bukkit item dropping code is still reliant on vanilla code to drop
   items and that vanilla code still sometimes fails to drop items even though
   the drop probability is 100%.  The work-around code still corrects that issue.