Test1.php

<?php

$a = array('rund' => '2', 'group' => '3', 'kupon' => 'utre', 'tralala' => 'shtur_kupon');
$b = array('grund' => '2', 'group' => 'ww', 'soup' => '1', 'tralala' => 'fd');

function two_arrays_merge_all_values(array $a, array $b) {
    foreach ($b as $b_key => $b_value) {
        $last_index_a = count($a)-1;
        $current_index = 0;
        foreach ($a as $a_key => $a_value) {
            if ($a_key === $b_key) {
                $unique = uniqid();
                $a[$b_key . '_' . $unique] = $b[$b_key];
                unset($b[$b_key]);
                break;
            }

            if ($current_index == $last_index_a) {
                $a[$b_key] = $b[$b_key];
                unset($b[$b_key]);
            }

            $current_index++;
        }
    }
    
    return $a;
}

$result = two_arrays_merge_all_values($a, $b);


print_r($result);