Here's an out-of-the-box option, using Racket's pattern matching<http://docs.racket-lang.org/reference/match.html#%28form._%28%28lib._racket%2Fmatch..rkt%29._define%2Fmatch%29%29> with the (? predicate) form:
(define/match (dup a) [((? string?)) (string-append a a)] [((? integer?)) (list a a)]) Dan
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