My duplicates are generally with all the other undisplayed Lego. In tubs or boxes
If you have the space, try making a few dioramas with your duplicates and taking some photos. Who doesn't want to see an unlikely title fight between Superman and MODOK?

If you're strapped for space Minifigs always sell for a pretty penny. I'd be in the market for a Hulk and Star Lord

Alternatively you can always be charitable. If you know any kids you'd make their day with a free Lego minifigure.