John Thingstad wrote:

It is the trivial to check for duplicates and lookup key

(defun duplicate-value-check ()
(let* ((values (loop for v in (cdr *enum*) by #'cddr collect v))
(sorted-values (sort values #'<))
(position (mismatch sorted-values
(remove-duplicates sorted-values))))
(when position
(let ((index (nth position sorted-values)))
(error "key ~A has a value ~D duplicated"
(nth (* index 2) *enum*) (nth index sorted-values))))))

Testing with ABCL:

(setq *enum* '(a 2 b 3 c 4 d 5 e 6 f 7 g 8 h 9 i 2))
(duplicate-value-check)

CL-USER(3): Debugger invoked on condition of type SIMPLE-ERROR:
key C has a value 3 duplicated

It should have been:
key i has a value 2 duplicated

Gauche Scheme

(define (dup-check plist)
(let1 seen '()
(do-plist ((k v) plist)
(if (member v seen)
(errorf "Key ~a has dup. value ~a." k v)
(push! seen v)))))

(dup-check '(a 2 b 3 c 4 d 3 e 6 f 7 g 8 h 3 i 42))

*** ERROR: Key d has dup. value 3.

--
[T]he problem is that lispniks are as cultish as any other devout group and basically fall down frothing at the mouth if they see [heterodoxy].
--- Kenny Tilton
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham

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