Use this code snippet such that any time you duplicate field id "950", it will prepend "Duplicate of " in front of the entry value.

add_filter('frm_before_duplicate_entry_values', 'before_duplicate_entry');
function before_duplicate_entry( $metas ) {
	$target_field_id = 950; // Replace 950 with your field ID.
	foreach ( $metas as $meta ) {
		if ( $target_field_id === (int) $meta->field_id ) {
			$meta->meta_value = 'Duplicate of ' . $meta->meta_value;
			break;
		}
	}
	return $metas;
}