diff --git a/guix/build/union.scm b/guix/build/union.scm
index ccd2d5c103..6640b56523 100644
--- a/guix/build/union.scm
+++ b/guix/build/union.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014 Mark H Weaver <mhw@netris.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -53,22 +53,24 @@
 identical, #f otherwise."
   (let ((st1 (stat file1))
         (st2 (stat file2)))
-    (and (eq? (stat:type st1) 'regular)
-         (eq? (stat:type st2) 'regular)
-         (= (stat:size st1) (stat:size st2))
-         (call-with-input-file file1
-           (lambda (port1)
-             (call-with-input-file file2
-               (lambda (port2)
-                 (define len 8192)
-                 (define buf1 (make-bytevector len))
-                 (define buf2 (make-bytevector len))
-                 (let loop ()
-                   (let ((n1 (get-bytevector-n! port1 buf1 0 len))
-                         (n2 (get-bytevector-n! port2 buf2 0 len)))
-                     (and (equal? n1 n2)
-                          (or (eof-object? n1)
-                              (loop))))))))))))
+    ;; When deduplication is enabled, identical files share the same inode.
+    (or (= (stat:ino st1) (stat:ino st2))
+        (and (eq? (stat:type st1) 'regular)
+             (eq? (stat:type st2) 'regular)
+             (= (stat:size st1) (stat:size st2))
+             (call-with-input-file file1
+               (lambda (port1)
+                 (call-with-input-file file2
+                   (lambda (port2)
+                     (define len 8192)
+                     (define buf1 (make-bytevector len))
+                     (define buf2 (make-bytevector len))
+                     (let loop ()
+                       (let ((n1 (get-bytevector-n! port1 buf1 0 len))
+                             (n2 (get-bytevector-n! port2 buf2 0 len)))
+                         (and (equal? n1 n2)
+                              (or (eof-object? n1)
+                                  (loop)))))))))))))
 
 (define* (union-build output inputs
                       #:key (log-port (current-error-port)))