Fix "Failed to add path to set" error during nixos-rebuild

pkgs/os-specific/linux/systemd/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation rec {
     sha256 = "0cgnnl6kqaz3als5y9g8jvsvbs4c8ccp0vl4s1g8rwk69w2cwxd2";
   };
 
-  patches = [ ./reexec.patch ];
+  patches = [ ./reexec.patch ./ignore-duplicates.patch ];
 
   buildInputs =
     [ pkgconfig intltool gperf libcap dbus kmod xz pam acl

pkgs/os-specific/linux/systemd/ignore-duplicates.patch

@@ -0,0 +1,20 @@
+Ignore duplicate paths in "systemctl start".
+
+diff --git a/src/systemctl/systemctl.c b/src/systemctl/systemctl.c
+index a4290c4..d3ac3ad 100644
+--- a/src/systemctl/systemctl.c
++++ b/src/systemctl/systemctl.c
+@@ -1596,8 +1596,11 @@ static int start_unit_one(
+ 
+                 r = set_put(s, p);
+                 if (r < 0) {
+-                        log_error("Failed to add path to set.");
+-                        return r;
++                        free(p);
++                        if (r != -EEXIST) {
++                                log_error("Failed to add path %s to set.", p);
++                                return r;
++                        }
+                 }
+ 
+                 p = NULL;