When an object uses linked mesh data and the object and the modifier
node group are made local after versioning runs for the first time,
a new modifier is added every time the file is opened. An obvious
fix is to check whether there is already a modifier with the versioning
node group. In that case we can skip the object.
This doesn't fully fix the scenario from #120030; it requires that
the node group is made local so it doesn't leave a reference to
a non-existent library data-block.
7ba8fc1768