When I am changing owner of opportunity that time when send notification email checkbox is checked then new owner gets notification twice. there is no any process builder or workflow which triggers it. please help,
Check all the workflow or Process builder which are responsible for sending the notification email. Or check your condition if it has OR condition than there might be some chances that it will get triggered twice. And best way to find out is to check the debug logs. There you can check and find if it is trigger twice. if possible can you post whatever you have done so far?