Use identifier-binding to check if an identifier is defined or not

check-is-defined.rkt

#lang racket

(require (for-syntax racket/syntax
                     syntax/parse))

(module+ test
  (require rackunit)
  (let ()
    (define/if-not-exists t 0)
    (check-equal? t 0))
  (check-not-exn
    (lambda ()
      (define/if-not-exists t #t)
      (define/if-not-exists t '())
      (check-equal? t #t))))
  

(define-syntax (define/if-not-exists stx)
  (syntax-parse stx
    [(_ x:id value:expr)
      #:when (not (identifier-binding #'x))
      (syntax/loc stx (define x value))]
    [(_ x:id value:expr)
     (syntax/loc stx (void))]))