Use this example to add three underscores as the field key separator. For example, if you duplicate a field, a result could be fieldkey2. With the custom separator, you can change it so it would become fieldkey___2.

add_filter( 'frm_unique_field_key_separator', 'use_custom_field_key_separator' );
function use_custom_field_key_separator() {
	return '___';
}