[danieltillett]

This is a bit different in that Greece has control over a real printing press. While it would be unlikely to be kept hidden, it should be possible for the Greek printers to avoid overlapping serials numbers.

In practice they could only start their own “unauthorised” printing if it was kept secret since if it leaked the ECB would just issue new notes. This would likely be highly disruptive to the ECB as it was to Portugal.