Preventing Laravel adding multiple records to a pivot table

@alexandre Butynsky method works very well but use two sql queries.

One to check if cart contains the item and one to save.

To use only one query use this:

try {
    $cart->items()->save($newItem);
}
catch(\Exception $e) {}

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 :)

You can check the presence of an existing record by writing a very simple condition like this one :

if (! $cart->items->contains($newItem->id)) {
    $cart->items()->save($newItem);
}

Or/and you can add unicity condition in your database, it would throw an exception during an attempt of saving a doublet.

You should also take a look at the more straightforward answer from Barryvdh.