My naive looked more like: (with apologies on likely messing up spacing in hn.) (let ((in '((1 "one") (1 "two") (1 "three") (1 "one") (1 "four") (1 "two"))

My naive looked more like: (with apologies on likely messing up spacing in hn.) (let ((in '((1 "one") (1 "two") (1 "three") (1 "one") (1 "four") (1 "two")))) (loop with hash = (make-hash-table :test #'equal) for v in in do (incf (gethash (cadr v) hash 0)) finally (return (loop for key being the hash-keys of hash collect (list (gethash key hash) key)))))