diff --git a/nix/nix-daemon/guix-daemon.cc b/nix/nix-daemon/guix-daemon.cc
index 0bb9f7559c..0309743de1 100644
--- a/nix/nix-daemon/guix-daemon.cc
+++ b/nix/nix-daemon/guix-daemon.cc
@@ -249,6 +249,9 @@ main (int argc, char *argv[])
   settings.useChroot = false;
 #endif
 
+  /* Turn automatic deduplication on by default.  */
+  settings.autoOptimiseStore = true;
+
   argvSaved = argv;
 
   try
@@ -325,6 +328,10 @@ main (int argc, char *argv[])
 	}
 #endif
 
+      printMsg (lvlDebug,
+		format ("automatic deduplication set to %1%")
+		% settings.autoOptimiseStore);
+
       printMsg (lvlDebug,
 		format ("listening on `%1%'") % settings.nixDaemonSocketFile);
 
diff --git a/tests/derivations.scm b/tests/derivations.scm
index 19bcebcb21..855b059d16 100644
--- a/tests/derivations.scm
+++ b/tests/derivations.scm
@@ -151,6 +151,28 @@ (define prefix-len (string-length dir))
          ;; the contents.
          (valid-path? %store (derivation->output-path drv)))))
 
+(test-assert "identical files are deduplicated"
+  (let* ((build1  (add-text-to-store %store "one.sh"
+                                     "echo hello, world > \"$out\"\n"
+                                     '()))
+         (build2  (add-text-to-store %store "two.sh"
+                                     "# Hey!\necho hello, world > \"$out\"\n"
+                                     '()))
+         (drv1    (derivation %store "foo"
+                              %bash `(,build1)
+                              #:inputs `((,%bash) (,build1))))
+         (drv2    (derivation %store "bar"
+                              %bash `(,build2)
+                              #:inputs `((,%bash) (,build2)))))
+    (and (build-derivations %store (list drv1 drv2))
+         (let ((file1 (derivation->output-path drv1))
+               (file2 (derivation->output-path drv2)))
+           (and (valid-path? %store file1) (valid-path? %store file2)
+                (string=? (call-with-input-file file1 get-string-all)
+                          "hello, world\n")
+                (= (stat:ino (lstat file1))
+                   (stat:ino (lstat file2))))))))
+
 (test-assert "fixed-output-derivation?"
   (let* ((builder    (add-text-to-store %store "my-fixed-builder.sh"
                                         "echo -n hello > $out" '()))