There is much to do with duplicate legendaries. The Law of Kulle is a recipe for Kanai's cube that allows you to reroll the stats of a legendary item, so it's in your best interest to maintain a duplicate set of your gear to reroll when needed. Also, certain legendaries are universally useful, and keeping a few copies around to reroll can help.
You should hold onto at least a pair of :
- Endless walk
- Compass rose
- Convection of elements
- Obsidian ring of the Zodiac
- Warzechian Armguards
- Nemesis Bracers
- (Maybe) Litany of the Undaunted & the wailing host
- Avarice band
These are legendaries that are commonly called for in builds, (the bracers in particular are in literally every speed running build) and having them on hand, even if the stats aren't perfect, gives you a major leg up whilst transitioning gear.
NEVER EVER EVER disenchant or throw out either a Puzzle Ring nor a Bovine Bardiche, as they can be placed in the cube to open a portal to bonus levels.
You can also strip the legendary powers of most legendaries by putting them in the cube, allowing you to use them without having the item equipped.
Rares? Rares are trash. Salvage them, or, one you have 10k crafting mats, sell them.