Adds a check to the define/extend language parser

collects/redex/private/reduction-semantics.rkt

@@ -1743,7 +1743,17 @@
                           stx (or delim (car left))))
     (check-each prods delim? "expected production")
     (cons names prods))
-  (map parse-non-terminal (syntax->list nt-defs)))
+  (define parsed (map parse-non-terminal (syntax->list nt-defs)))
+  (define defs (make-hash))
+  (for ([p parsed])
+    (define ns (car p))
+    (for ([n ns])
+      (define m (hash-ref defs (syntax-e n) #f))
+      (if m
+          (raise-syntax-error #f "same non-terminal defined twice"
+                              stx n (list m))
+          (hash-set! defs (syntax-e n) n))))
+  parsed)
 
 (define-syntax (define-language stx)
   (syntax-case stx ()

collects/redex/tests/tl-test.rkt

@@ -1468,6 +1468,12 @@
                   (define-language good-lang (a 1 2))
                   (define-extended-language bad-lang5 good-lang (a) (b 2)))
                 #rx"at least one production")
+  (test-syn-err (define-language bad-lang5 (x 1) (x 2)) #rx"same non-terminal" 2)
+  (test-syn-err (define-language bad-lang6 ((x x) 1)) #rx"same non-terminal" 2)
+  (test-syn-err (let ()
+                  (define-language good-lang)
+                  (define-extended-language bad-lang7 good-lang ((x x) 1)))
+                #rx"same non-terminal" 2)
   
   (test-syn-err (redex-match grammar m_1) #rx"before underscore")
   (test-syn-err (redex-match grammar (variable-except a 2 c)) #rx"expected an identifier")