diff --git a/gnu/system.scm b/gnu/system.scm
index a51b176640..1a0aee41eb 100644
--- a/gnu/system.scm
+++ b/gnu/system.scm
@@ -511,9 +511,8 @@ (define (operating-system-etc-directory os)
       ((services     (operating-system-services os))
        (pam-services ->
                      ;; Services known to PAM.
-                     (delete-duplicates
-                      (append (operating-system-pam-services os)
-                              (append-map service-pam-services services))))
+                     (append (operating-system-pam-services os)
+                             (append-map service-pam-services services)))
        (profile-drv (operating-system-profile os))
        (skeletons   (operating-system-skeletons os))
        (/etc/hosts  (or (operating-system-hosts-file os)
diff --git a/gnu/system/linux.scm b/gnu/system/linux.scm
index 8cddedf28e..aaaa8c6dee 100644
--- a/gnu/system/linux.scm
+++ b/gnu/system/linux.scm
@@ -100,13 +100,19 @@ (define (pam-services->directory services)
                              services))))
     (define builder
       #~(begin
-          (use-modules (ice-9 match))
+          (use-modules (ice-9 match)
+                       (srfi srfi-1))
 
           (mkdir #$output)
           (for-each (match-lambda
                      ((name file)
                       (symlink file (string-append #$output "/" name))))
-                    '#$(zip names files))))
+
+                    ;; Since <pam-service> objects cannot be compared with
+                    ;; 'equal?' since they contain gexps, which contain
+                    ;; closures, use 'delete-duplicates' on the build-side
+                    ;; instead.  See <http://bugs.gnu.org/20037>.
+                    (delete-duplicates '#$(zip names files)))))
 
     (gexp->derivation "pam.d" builder)))