In Godot 4 C# Action Adventure: Build your own 2.5D RPG, in the “Adding Player Animations” video, it says that you can do command-c, command-v to copy keyframes on a Mac, but I don’t think that works. It seems like you have to put the bar on the first keyframe, hit command+c, move the bar to the new position, then go back to the first keyframe, right-click and choose “duplicate.”
Edit: I think I might have misheard it, and it’s actually supposed to be command-d, not command-v.
In case anyone else gets stuck there, that worked for me.