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#lang racket
(define/contract (unique-occurrences arr)
(-> (listof exact-integer?) boolean?)
(define occurrences (make-hash))
(for ([n (in-list arr)])
(hash-update! occurrences n add1 1))
(equal? (hash-values occurrences)
(remove-duplicates (hash-values occurrences))))
(unique-occurrences '(1 2 2 1 1 3))
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