Mizore

The reason you are getting two messages is because it's sending the chat to both the server side and the client side. You need to make sure it only sends it to one side (server side in most cases...). Anywhere where it sends a chat to the player, surround the statement (or multiple statements) with this statement: if(!world.isRemote){}

The motionX variable is a double. You are adding an integer to a double (which works, but gives the double the precision of an integer. You don't want that, you want the precision of a double. That's the entire reason the variable is a double . You need to specify the type in which you are adding to the double. I am willing to bet if you change: motionX += 1 to: motionX += 1.0D then it will be smoother. (Very very important in programming if you ever need anything to be precise that you specify the type.)

When setting a PlayerInteractEvent to cancelled, the player is able to continue breaking the block. Once the block is destroyed, the block disappears (allowing the player to momentarily walk into the block), and then re-appears again moments later. If the event is on Player Interaction, then it should stop the event as the player is still breaking the block correct? It may not be a bug, perhaps that is as the Forge creators intended it to be. I don't really know, that's why I am reporting this.