Spawning an entity produces the enitity and it's 'ghost' version by default. I make a mod in Single Player mode, so can't tell if the same is for multiplayer. For example spawn Coal gem, it's doubled but you can pick only one while the other is not interactive.
I found where the bug is: the spawn function should be surrounded by if(!world.isRemote) otherwise it makes a valid object on server side and ghost on client side (in single player theres a server too O.o, didn't knew that).
As this is my first post in here, and I'm very beginner in modding MC I'd like to say Hi and thanks for amazing work you did with MCreator. Probably you'll see me often in bug section =)