;;With loop:

(defun pack(lst &optional groups)
  (loop for el in lst 
        for first-group = (when (equal el (caar groups)) (pop groups))
        do (push (cons el first-group) groups))
  (reverse groups))

;;With recursion:

(defun pack(lst &optional groups)
  (if (not lst) (reverse groups) 
      (let* ((el (pop lst))
             (first-group (when (equal el (caar groups)) (pop groups))))
        (pack lst (cons (cons el first-group) groups)))))

;;With no mercy:

(defun pack(lst &optional g)
  (if (not lst) (reverse g) 
      (pack (cdr lst) (cons (cons (car lst) (when (equal (car lst) (caar g)) (pop g))) g)))))