diff --git a/collects/setup/setup-unit.ss b/collects/setup/setup-unit.ss
index 5a02a0d1db..6930bb7d28 100644
--- a/collects/setup/setup-unit.ss
+++ b/collects/setup/setup-unit.ss
@@ -305,90 +305,28 @@
             subs))))
 
   (define (check-again-all given-ccs)
-    ;; the function below has two parts: the first (doubly) commented part is
-    ;; od code that relies on having an identity for a directory, and cannot be
-    ;; used as explained there.  The solution was to put "marker" files to
-    ;; identify directories, but that doesn't work out too, since it requires
-    ;; writing into the directories which might not be possible and not needed.
-    ;; Specifically, installing planet libraries calls setup on
-    ;; scribblings/main/user -- but there's no need to write in there.  (This
-    ;; is especially bad on vista which creates a virtual user directory...)
-    ;; So the whole thing is disabled for now, and we plan to add a new new
-    ;; system level function for getting the identity of a file or a directory
-    ;; and use the original code here.
-    given-ccs)
-  #;
-  (define (check-again-all given-ccs)
-    #|
-    ;; This code is better than using marker files, but an older version of it
-    ;; relied on the obligatory existence of an "info.ss" file to implement
-    ;; `file-or-directory-identity'.  That is no longer required, so it needs
-    ;; to identify directories and that is currently not available.  So use the
-    ;; code below it instead.  Perhaps there will be some robust way to do this
-    ;; in the future, eg -- for directories, use the identity of their first
-    ;; file.
-    (define all-cc+ids
-      (map (lambda (cc)
-             (cons cc (file-or-directory-identity (cc-path cc))))
-           (plt-collection-closure all-collections)))
-    (for ([cc given-ccs])
-      (define given-id
-        (file-or-directory-identity (cc-path cc)))
-      (for ([found-cc+id all-cc+ids]
-            #:when (not (same-collection-name? cc (car found-cc+id))))
-        (when (eq? (cdr found-cc+id) given-id)
-          (error 'setup-plt
-                 "given collection path: ~e refers to the same info file as another path: ~e"
-                 (apply build-path (cc-collection cc))
-                 (apply build-path (cc-collection (car found-cc+id)))))))
-    |#
-    ;; Note: this is not a locking mechanism; specifically, if we find a marker
-    ;; file we assume that we generated it rather than another setup-plt
-    ;; process
-    (define all-ccs (plt-collection-closure all-collections))
-    (define (cc->name cc)
-      (string-join (map path->string (cc-collection cc)) "/"))
-    (define all-names   (map cc->name all-ccs))
-    (define given-names (map cc->name given-ccs))
-    (define (cc-mark cc) (build-path (cc-path cc) ".setup-plt-marker"))
-    (define (complain-about-mark name mark)
-      (let ([given (with-handlers ([void (lambda (_) '???)])
-                     (with-input-from-file mark read-line))])
-        (error 'setup-plt
-               "given collection path: \"~a\" refers to the same directory as another given collection path, \"~a\""
-               name given)))
-    ;; For cleanup: try to remove all files, be silent
-    (define (cleanup)
-      (for ([cc (append given-ccs all-ccs)])
-        (let ([mark (cc-mark cc)])
-          (when (file-exists? mark)
-            (with-handlers ([void void]) (delete-file mark))))))
-    ;; First remove all marker files if any, let it fail if we can't remove it
-    (define (remove-markers)
-      (for ([cc given-ccs])
-        (let ([mark (cc-mark cc)])
-          (when (file-exists? mark)
-            (setup-printf "WARNING"
-                          "found a marker file, deleting: ~a"
-                          (path->name mark))
-            (delete-file mark)))))
-    ;; Now create all marker files, signalling an error if duplicate
-    (define (put-markers)
-      (for ([cc given-ccs] [name given-names])
-        (let ([mark (cc-mark cc)])
-          (if (file-exists? mark)
-            (complain-about-mark name mark)
-            (with-output-to-file mark (lambda () (printf "~a\n" name)))))))
-    ;; Finally scan all ccs and look for duplicates
-    (define (scan-all)
-      (for ([cc all-ccs] [name all-names])
-        (when (and (not (member name given-names))
-                   (file-exists? (cc-mark cc)))
-          (complain-about-mark name (cc-mark cc)))))
-    (dynamic-wind
-      void
-      (lambda () (remove-markers) (put-markers) (scan-all) given-ccs)
-      cleanup))
+    (define (cc->cc+name+id cc)
+      (list cc
+            (string-join (map path->string (cc-collection cc)) "/")
+            (file-or-directory-identity (cc-path cc))))
+    (define all-ccs+names+ids
+      (map cc->cc+name+id (plt-collection-closure all-collections)))
+    (define given-ccs+names+ids
+      (remove-duplicates
+       (map cc->cc+name+id (plt-collection-closure given-ccs))
+       (lambda (x y) (equal? (cadr x) (cadr y)))))
+    (for ([given-cc+name+id (in-list given-ccs+names+ids)])
+      (cond
+        [(ormap (lambda (cc+name+id)
+                  (and (not (equal? (cadr cc+name+id) (cadr given-cc+name+id)))
+                       (equal? (caddr cc+name+id) (caddr given-cc+name+id))
+                       (cadr cc+name+id)))
+                all-ccs+names+ids)
+         => (lambda (bad)
+              (error 'setup-plt
+                     "given collection path: \"~a\" refers to the same directory as another given collection path, \"~a\""
+                     (cadr given-cc+name+id) bad))]))
+    (map car given-ccs+names+ids))
 
   (define (sort-collections ccs)
     (sort ccs string<? #:key cc-name))