If a function call has two pairs of keyword arguments with the same keyword, the leftmost argument pair is used. This can be helpful if a list of keyword arguments is received and you want to selectively override some value in that argument list in application. There’s no need to use mutation or produce a fresh variation of the list.
(defun write-escaped (object &rest args &key &allow-other-keys) (apply 'write object :escape t args)) * (write "hello" :escape nil) hello * (write-escaped "hello" :escape nil) "hello"
See 188.8.131.52 for details.