If I write a flow and export it then import it to another machine, all the existing nodes are honoured and not overwritten.
Alas I have seen that if you write a subflow and export a flow with it then import it to another machine which already has that subflow, a second (or third, etc) copy of the subflow is created.
I get that subflows can be different between machines. Though I can't exactly see why.
They are like nodes.
If you have a different version of NR on one machine to another when you copy flows, they don't take their most up to date (or older) nodes with them.
If the two (sub)flows are different, then maybe make another version of it on the second machine.
Otherwise, I can't see why it is creating a copy.