Use this example to add three underscores as the form key separator. For example, if you duplicate a form, a result could be formkey2. With the custom separator, you can change it so it would become formkey___2.

add_filter( 'frm_unique_form_key_separator', 'use_custom_form_key_separator' );
function use_custom_form_key_separator() {
	return '___';
}