If I duplicated an object, and slightly edited the shape of the duplicate, and want to make all of the other objects look just like the edited one, how do I do that?
Select both, last one ( the active object, yellow outline instead of orange ) will be the source, ctrl + l > object data. Note it will link the geometry, so if you modify one, it’ll modify the other too. You can break that link by pressing U > make single user.
Note that you can use alt D for duplicate linked instead of the simple duplicate too.