diff --git a/registries_conf_ctl/cli.py b/registries_conf_ctl/cli.py
index 060d8b5..fe55cec 100644
--- a/registries_conf_ctl/cli.py
+++ b/registries_conf_ctl/cli.py
@@ -86,7 +86,7 @@ def _extend(d, key, what):
     if key not in d:
         d[key] = [what]
     else:
-        d[key].add(what)
+        d[key] = list(set(d[key] + [what]))
 
 
 class DockerDaemonJson(Fmt):
diff --git a/tests/test_add_mirror.py b/tests/test_add_mirror.py
index 285eb27..17254bc 100644
--- a/tests/test_add_mirror.py
+++ b/tests/test_add_mirror.py
@@ -95,9 +95,14 @@ def test_add_mirror(test_input, expected, cls, tmpdir):
     fmt.add_mirror('docker.io', 'vossi04.front.sepia.ceph.com:5000', True, True)
     assert fmt.config == expected
 
+
     p = tmpdir.join("conf.conf")
     p.write(test_input)
 
     subprocess.check_call('registries-conf-ctl --conf {p} add-mirror docker.io vossi04.front.sepia.ceph.com:5000 --insecure --http'.format(p=p), shell=True)
 
     assert cls(p).config == expected
+
+    subprocess.check_call('registries-conf-ctl --conf {p} add-mirror docker.io vossi04.front.sepia.ceph.com:5000 --insecure --http'.format(p=p), shell=True)
+
+    assert cls(p).config == expected