cmd + e will mark the current selection (or current word if there's no selection), cmd + g (or cmd + shift + g to go backwards) will find other instances of it.

cmd + e will mark the current selection (or current word if there's no selection), cmd + g (or cmd + shift + g to go backwards) will find other instances of it. ctrl + cmd + g will find all instances of the current word, which you can edit simultaneously.