First, create map with several new nodes and don't change the default names. Then assign different spaces to nodes but set to "New node" and "New Node 2" one space (e.g. "Main"). Now you can change node names in spaces so you will have several "New node" nodes in different spaces. If you will select all "New node" nodes and will try to assign then existing space but not the space that contains "New node 2" (e.g. "Main") constraint exception generated.

Uniquieness of the node name is ensured in concrete spaces (you can have several nodes with the same name in the different spaces but not in the one space). The problem is in successive assigning name and space properties of the node. When unique name is generated for the new space it can't be assigned for the node that still has the old space because there might be a node with the same name.