You want to add element to the list, but make all elements unique. So it work like set.
(define (adjoin x a) (if (member x a) a (cons x a)))
Credit: Nils M Holm (ref: adjoin.scm)
(adjoin 'x '(a b c)) ;; ==> (x a b c) (adjoin 'c '(a b c)) ;; ==> (a b c)
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