When I started my O27 layout and I needed at least 4 locos to run on my two relayed loops. 2-4-2 locos, some with plastic bodies, were used. I upgraded to four 2026 locos and then four 2035s. I will pick up spare or extra locos or LWs transformers if the price is right, like the LW I got for $10 at my last train show.
I do not buy duplicates as such. I like to acquire different pieces from what I have instead. I am limited by size as all gear must be able to run on O27 track and switches.
But I love duplicates of favorite tools, cameras, golf clubs, etc. For tools I keep temporarily misplacing them and cameras and golf clubs I break them. For tools I have 4 work areas where some are required plus cars.
Charlie