diff --git a/paramiko/client.py b/paramiko/client.py
index 5b71958..4fa28e5 100644
--- a/paramiko/client.py
+++ b/paramiko/client.py
@@ -186,6 +186,12 @@ class SSHClient (object):
 
         @raise IOError: if the file could not be written
         """
+
+        # update local host keys from file (in case other SSH clients
+        # have written to the known_hosts file meanwhile.
+        if self.known_hosts is not None:
+            self.load_host_keys(self.known_hosts)
+
         f = open(filename, 'w')
         f.write('# SSH host keys collected by paramiko\n')
         for hostname, keys in self._host_keys.iteritems():