Preventing Laravel adding multiple records to a pivot table

You can also use the $model->sync(array $ids, $detaching = true) method and disable detaching (the second param).

$cart->items()->sync([$item->id], false);

Update:
Since Laravel 5.3 or 5.2.44, you can also call syncWithoutDetaching:

$cart->items()->syncWithoutDetaching([$item->id]);

Which does exactly the same, but more readable 🙂