diff --git a/ice-9/session.scm b/ice-9/session.scm
index 5901eba23..208230938 100644
--- a/ice-9/session.scm
+++ b/ice-9/session.scm
@@ -264,12 +264,18 @@ The forest traversed is the image of the forest generated by root
 modules returned by INIT-THUNK and the generator TRAVERSE.
 It is an image under the mapping EXTRACT."
   (lambda (fold-module init)
-    (let rec ((data init)
-	      (modules (init-thunk)))
-      (do ((modules modules (cdr modules))
-	   (data data (rec (fold-module (extract (car modules)) data)
-			   (traverse (car modules)))))
-	  ((null? modules) data)))))
+    (let* ((table (make-hash-table 31))
+	   (first? (lambda (obj)
+		     (and (not (hash-ref table obj))
+			  (hash-create-handle! table obj #t)))))
+      (let rec ((data init)
+		(modules (init-thunk)))
+	(do ((modules modules (cdr modules))
+	     (data data (if (first? (car modules))
+			    (rec (fold-module (extract (car modules)) data)
+				 (traverse (car modules)))
+			    data)))
+	    ((null? modules) data))))))
 
 (define-public (apropos-fold-accessible module)
   (make-fold-modules (lambda () (list module))