Tableau

$entrees = array('la', 'dans', 'entre', 'la', 'la', 'ou', 'dans');

$entreesEnDouble = array();

$sorties = array();

foreach($entrees as $key => $value) {
    unset($entrees[$key]);
    
    if (in_array($value, $entrees) || in_array($value, $entreesEnDouble)) {
        $entreesEnDouble[] = $value;        
    } else {
        $sorties[] = $value;
    }
}

var_dump($sorties); // array(2) { [0]=> string(5) "entre" [1]=> string(2) "ou" }