In emacs, C-_ is undo. If you perform a non-undo operation, then undo it, then do a non-undo operation, then two undos will undo the undo.

Unless you’re using one of emacs’s alternate undo systems, like undo-fu, undo-tree, or vundo, which may have their own ways of doing things.

I use undo-tree, and with the default bindings there, there’s an undo on C-_ and a redo on M-_; undo doesn’t undo undos there.