With ASP.Net, I need to submit twice when using the RequiredFieldValidator in a FCKeditor instance FCKeditor will not work properly with the Required Field Validator when the "EnableClientScript" property of the validator is set to "true" (default). Due to a limitation in the default validation system, you must set it to "false".
If you want to do client side validation, you must use a Custom Validator instead and provide the appropriate validation function, using the FCKeditor JavaScript API.