;; ERROR: The variable #:G1 occurs more than once in the lambda list.
(let (x)
  (symbol-macroletf ((y x))
    (setf (values y y) t)))

;; This change fixes it.
(define-setf-expander symbol-macroletf-helper (store-vars writer-form
                                               reader-form)
  (let* ((new-store-vars (loop repeat (length store-vars)
                               collect (gensym)))
         (new-writer-form `(let (,@(mapcar #'list store-vars new-store-vars))
                             ,writer-form)))
    (values '() '() new-store-vars new-writer-form reader-form)))