Oh, I don’t love having duplicates at all. I simply have over 13k duplicated cards because I’m too lazy to sell them. 
If someone is willing to buy all my duplicated cards for ~60% market value, you can buy it right now, haha.
As a collector, I also prefer to have no duplicated cards at all. The only cards I specifically have duplicates of, are those that overlap in my main collections (e.g. the English 6/34 Team Aqua’s Kyogre full art from the Double Crisis set I have two of: one in my Seviper collection due to its Seviper cameo; and one in my FA Supporter/CHR collection. Likewise for these seven FA Pikachu cards that have trainers in the background. And there are a couple Pikachu cards I have in both my Pikachu collection and ‘first TCG release of each language’ collection (four to be exact).
I think those are the only cards I wanted to have duplicates of. 
I still enjoy the sight of having loads of the same artwork as entire binders full, but for that I have my many languages and variations like non-holo vs RH; unlimited vs 1st edition; stamped cards; etc. 
Greetz,
Quuador