Copy/Paste and Import/Export are two very different actions.
When you copy an instance of the subflow, you are only copying the instance node. When you paste, you are pasting a copy of just the instance node.
When you export the Instance node, you are not only exporting the instance, but the full subflow definition to go with it.
When you import that flow, you are importing the subflow definition and the instance. Node-RED detects you are importing what appears to be a duplicate of a subflow you already have. The current behaviour is to import it as a distinct subflow - because that the easiest thing to do.
There is a plan to do a better job of this in the future - to let the user choose whether to import it as a copy, use the existing subflow definition, or replace the existing definition with the one being imported.
