Laravel 5.4, allow the user to only favourite something once so it does not get added to their list twice?

I have a working favourite system in my application, that allows the users' to favourite a charity and these favourites are displayed to them on another page.

The only problem is that the user is able to favourite the same charity multiple times, how could I limit this to just once and then maybe tell the user that they already have favourited this?

via Chebli Mohamed