Repeat functionality is provided by the repeat.el Emacs Lisp package, which is included with standard Emacs distributions. From repeat.el‘s documentation:
This package defines a command that
repeats the preceding command,
whatever that was, including its
arguments, whatever they were. This
command is connected to the key C-x z.
To repeat the previous command once,
type C-x z. To repeat it a second time
immediately after, type just z. By
typing z again and again, you can
repeat the command over and over.
To see additional information about the repeat command, type C-h F repeat RET from within Emacs.