Don't add keys that are already there.

Author: rohe

src/cryptojwt/key_bundle.py

@@ -208,7 +208,8 @@ def do_keys(self, keys):
                     except JWKException as err:
                         logger.warning('While loading keys: {}'.format(err))
                     else:
-                        self._keys.append(_key)
+                        if _key not in self._keys:
+                            self._keys.append(_key)
                         flag = 1
                         break
             if not flag: