This can cause duplication bugs when canceling this event to instead drop the block as a dropped itemstack.

To reproduce:

Use the following event: https://github.com/MLG-Fortress/HotFix/blob/b69fffb310f84b3847ff62f674cc4c293e29d7c7/src/main/java/me/robomwm/HotFix/Main.java#L39

1. Drop a block that can fall in an End Portal (Sand, Dragon egg, gravel, etc.)
2. Observe the debug message being printed twice in chat (or once if nobody's in The End and it's unloaded, and again when a player teleports through).

I'm not sure if this can be considered a spigot bug, or is instead a vanilla bug triggering this event to be fired twice.