diff --git a/main.py b/main.py
index 013752a..315052e 100644
--- a/main.py
+++ b/main.py
@@ -40,18 +40,29 @@ class AutoAddPolicy(paramiko.client.MissingHostKeyPolicy):
     """
     lock = threading.Lock()
 
+    def is_missing_host_keys(self, client, hostname, key):
+        k = client._host_keys.lookup(hostname)
+        if k is None:
+            return True
+        host_key = k.get(key.get_name(), None)
+        if host_key is None:
+            return True
+        if host_key != key:
+            raise paramiko.BadHostKeyException(hostname, key, host_key)
+
     def missing_host_key(self, client, hostname, key):
         with self.lock:
-            keytype = key.get_name()
-            logging.info(
-                'Adding {} host key for {}'.format(keytype, hostname)
-            )
-            client._host_keys.add(hostname, keytype,  key)
-
-            with open(client._host_keys_filename, 'a') as f:
-                f.write('{} {} {}\n'.format(
-                    hostname, keytype, key.get_base64()
-                ))
+            if self.is_missing_host_keys(client, hostname, key):
+                keytype = key.get_name()
+                logging.info(
+                    'Adding {} host key for {}'.format(keytype, hostname)
+                )
+                client._host_keys.add(hostname, keytype,  key)
+
+                with open(client._host_keys_filename, 'a') as f:
+                    f.write('{} {} {}\n'.format(
+                        hostname, keytype, key.get_base64()
+                    ))
 paramiko.client.AutoAddPolicy = AutoAddPolicy