Fix chestplate duplication

src/main/java/net/trueog/flyingbowog/Listeners/OnEntityShootBowEvent.java

@@ -18,6 +18,7 @@ import org.bukkit.event.EventHandler;
 import org.bukkit.event.Listener;
 import org.bukkit.event.entity.EntityShootBowEvent;
 import org.bukkit.inventory.ItemStack;
+import org.bukkit.inventory.PlayerInventory;
 import org.bukkit.potion.PotionData;
 import org.bukkit.potion.PotionType;
 
@@ -138,14 +139,32 @@ public class OnEntityShootBowEvent implements Listener {
         if (!(holdingFlyingBow || ridingFlyingBowArrow))
             return;
 
-        /* event is NOT cancellable in <1.20 Purpur builds, so revert the change */
+        /*
+         * Event is NOT cancellable in <1.20 Purpur builds, so revert by hand. The
+         * vanilla equip is a SWAP: the elytra moves into the chest slot and the old
+         * chestplate (oldItem) is pushed into the inventory slot the elytra came from.
+         * Naively calling setChestplate(oldItem) would leave TWO copies of the old
+         * chestplate -- the one we write into the chest slot plus the one vanilla
+         * already moved into the inventory -- duping it (and any other hotbar item
+         * swapped this way). Remove that displaced copy first.
+         */
         ItemStack previous = event.getOldItem();
         ItemStack elytra = newItem.clone();
 
         Bukkit.getScheduler().runTask(FlyingBowOG.getPlugin(), () -> {
 
-            player.getInventory().setChestplate(previous);
-            Map<Integer, ItemStack> leftovers = player.getInventory().addItem(elytra);
+            PlayerInventory inventory = player.getInventory();
+
+            // Drop the displaced copy the vanilla swap left in the inventory (if any)
+            // before restoring it to the chest slot, so we never end up with two.
+            if (previous != null && previous.getType() != Material.AIR)
+                inventory.removeItem(previous.clone());
+
+            // Restore the old chestplate to the chest slot (clears the elytra).
+            inventory.setChestplate(previous);
+
+            // Hand the elytra back; drop whatever does not fit.
+            Map<Integer, ItemStack> leftovers = inventory.addItem(elytra);
             leftovers.values().forEach(item -> player.getWorld().dropItemNaturally(player.getLocation(), item));
 
         });