diff --git a/letsencrypt-nginx/letsencrypt_nginx/parser.py b/letsencrypt-nginx/letsencrypt_nginx/parser.py
index fb79703dc..93cda2c41 100644
--- a/letsencrypt-nginx/letsencrypt_nginx/parser.py
+++ b/letsencrypt-nginx/letsencrypt_nginx/parser.py
@@ -257,6 +257,8 @@ class NginxParser(object):
 
         ..note :: If replace is True, this raises a misconfiguration error
         if the directive does not already exist.
+        ..note :: If replace is False nothing gets added if an identical
+        block exists already.
 
         ..todo :: Doesn't match server blocks whose server_name directives are
             split across multiple conf files.
@@ -480,7 +482,9 @@ def _add_directives(block, directives, replace=False):
         if not replace:
             # We insert new directives at the top of the block, mostly
             # to work around https://trac.nginx.org/nginx/ticket/810
-            block.insert(0, directive)
+            # Only add directive if its not already in the block
+            if directive not in block:
+                block.insert(0, directive)
         else:
             changed = False
             if len(directive) == 0:
diff --git a/letsencrypt-nginx/letsencrypt_nginx/tests/parser_test.py b/letsencrypt-nginx/letsencrypt_nginx/tests/parser_test.py
index b28640d7f..2d6156429 100644
--- a/letsencrypt-nginx/letsencrypt_nginx/tests/parser_test.py
+++ b/letsencrypt-nginx/letsencrypt_nginx/tests/parser_test.py
@@ -133,11 +133,11 @@ class NginxParserTest(util.NginxTest):
         self.assertEqual(1, len(re.findall(ssl_re, dump)))
 
         server_conf = nparser.abs_path('server.conf')
-        nparser.add_server_directives(server_conf,
-                                      set(['alias', 'another.alias',
-                                           'somename']),
+        names = set(['alias', 'another.alias', 'somename'])
+        nparser.add_server_directives(server_conf, names,
                                       [['foo', 'bar'], ['ssl_certificate',
                                                         '/etc/ssl/cert2.pem']])
+        nparser.add_server_directives(server_conf, names, [['foo', 'bar']])
         self.assertEqual(nparser.parsed[server_conf],
                          [['ssl_certificate', '/etc/ssl/cert2.pem'],
                           ['foo', 'bar'],