Boats can only die once.

In certain scenarios a boat can be killed multiple ways in a single tick.
Due to improper guards this can cause the death code to run multiple times
creating item drops. To correct this we insert the appropriate death check.

src/main/java/net/minecraft/server/EntityBoat.java

@@ -294,7 +294,7 @@ public class EntityBoat extends Entity {
 
             this.move(this.motX, this.motY, this.motZ);
             if (this.positionChanged && d3 > 0.2D) {
-                if (!this.world.isStatic) {
+                if (!this.world.isStatic && !this.dead) { // CraftBukkit - That which is dead cannot die
                     // CraftBukkit start
                     Vehicle vehicle = (Vehicle) this.getBukkitEntity();
                     VehicleDestroyEvent destroyEvent = new VehicleDestroyEvent(vehicle, null);