I think whether to get a duplicate depends on how much you love it, your financial resources, and your concern about availability of parts down the road.
For instance, I have duplicates of certain Postwar engines I love, such as the Lionel Berkshire #736, Lionel baby Hudson #2065, and Lionel ATSF 6220 bell-ringer switcher, all of which are relatively modest in cost when in good condition and good operating order.
On the other hand, I have an MTH Proto 3 NY Central RS3 that I am crazy about. I think it's my best locomotive, and I have thought about buying a duplicate, but it's about a $400 engine (not prohibitively expensive, but still more than I want to spend at this time for a duplicate).
Arnold