--- a/module/ice-9/psyntax.scm
+++ b/module/ice-9/psyntax.scm
@@ -296,9 +296,10 @@
 
       (define put-global-definition-hook
         (lambda (symbol type val)
-          (module-define! (current-module)
-                          symbol
-                          (make-syntax-transformer symbol type val))))
+          (unless (module-locally-bound? (current-module) symbol)
+            (module-define! (current-module)
+                            symbol
+                            (make-syntax-transformer symbol type val)))))
     
       (define get-global-definition-hook
         (lambda (symbol module)