cl-pushnew

cl-pushnew is a Lisp macro in `cl-lib.el'.

(cl-pushnew X PLACE [KEYWORD VALUE]...)

(cl-pushnew X PLACE): insert X at the head of the list if not already there.
Like (push X PLACE), except that the list is unmodified if X is `eql' to
an element already on the list.

Keywords supported: :test :test-not :key