<?php
//Trying to avoid 
/*
Class RainLab\User\Controllers\Users has already been extended with Backend\Behaviors\RelationController
/home/kurt/www/october/vendor/october/rain/src/Extension/ExtendableTrait.php line 133
*/

//by doing

        UsersController::extend(function (UsersController $controller) {                             // type hint for IDE mostly, but it's nice to type hint
            if (!in_array('Backend.Behaviors.RelationController', $controller->implement)) {         // check if that behavior is in public $implement variable already?
                $controller->implement[] = 'Backend.Behaviors.RelationController';                   // if it's not, add this behavior
                $controller->relationConfig = [];                                                    // RelationController behavior will crash if public $relationConfig is not set, use your desired configuration here
            }
        });
 
?>