Array
(
  [0] => Array
    (
        [name] => product_1
        [model] => model_1
        [option] => Array
            (
                [0] => Array
                    (
                        [product_id] => 47
                        [name] => Размер
                        [value] => 39
                    )
            )
        [quantity] => 10
    )
  [1] => Array
    (
        [name] => product_1
        [model] => model_1
        [option] => Array
            (
                [0] => Array
                    (
                        [product_id] => 47
                        [name] => Размер
                        [value] => 40
                    )
            )
        [quantity] => 20
    )
)
и т.д.

Array
(
  [0] => Array
    (
        [name] => product_1
        [model] => model_1
        [option] => Array
            (
                [0] => Array
                    (
                        [product_id] => 47
                        [name] => Размер
                        [value] => 39
                    )
                [1] => Array
                    (
                        [product_id] => 47
                        [name] => Размер
                        [value] => 40
                    )
            )
        [quantity] => 10
    )

$keys=array(); // Массив ключей, которые уже встречались
  foreach($products as $k=>$val) {
   if(array_key_exists($val['model'],$keys)) {
    unset($products[$k]);
   } else {
    $keys[$val['model']]=1;
   }
  }

$keys=array(); // Массив ключей, которые уже встречались
  foreach($products as $k=>$val) {
   if(array_key_exists($val['model'],$keys)) {
    unset($products[$k]);
   } else {
    $keys[$val['model']]=1;
   }
   if(isset($products[$k+1]['option']))
   {
    array_push($products[0]['option'], $products[$k+1]['option']);
   }
  }