This shouldn't happen, but if it does happen it'll definitely break the block.
We'll notify the server administrator on load and only keep the first block we
see.
Note that this will only happen if there's something seriously wrong with the
block as generally there'll not be a situation where information is stored in
two different files.
If incomplete blocks are loaded, without an "id", items will not have
any usable slimefun block information and will not be usable. Breaking
these blocks will give you a "CSCoreLib's Head".
CraftBukket's HumanEntity.closeInventory() apparently removes the human
from the inventory's viewer list, resulting in an exception when we were
calling closeInventory while iterating over the list of said viewers.
FixesTheBusyBiscuit/Slimefun4#364.