As an armor specialist, I look for variety and try to avoid duplications. It depends on if you consider variants within the same vehicle series to be duplicates or not, but I have a very short list of even that category:
2 Pz IIs (both 1/35 Tamiyas, one DAK, one BoF)
2 Pz IIIs (1/35 Tamiyas, one DAK, one BoF)
2 Pz IVs (1/35 Tamiya Ausf D BoF, 1/35 Academy H/J)
And that's it...all the others are one of a kind within their variant class although I will soon have two StuG III's, one a C/D and one a G, in the collection.