the error is caused because you have multiple instances of the Server Validations server behavior. You should only have one instance of the server validations server behavior on the page. The one server behavior can be set to validate all of the form elements.
Select the validation type, click the lightning bolt next to server variable and select the form element, click the plus button to add it, then set up the next form element validation and click the plus button. Repeat those steps for all the form elements.