prevent duplicate for Composite key columns

working environment: XAMPP on Windows 10

have Unique Index - Composite:

Code:

ALTER TABLE `sales`.`brand_categories`
  ADD  UNIQUE INDEX `ui_cat_brand` (`category_id` , `brand_id`);

when user will save a duplicate entry for the combination of both columns, database will throw error.
but is there a builtin validation rule which is suitable for it like is_unique.
if yes please guide How To.
if no builtin validation rule then how i can write a custome validation rule and prevent duplicate in this case
please help.

in Model have validation rule (this is just sample what i can do -as per my basic knowledge- for uniqueness of a single column)

Code:

protected $validationRules = [
        'category_name' =>[
            'rules' =>'required|is_unique[brand_categories.category_name,id,{id}]',
            'errors' =>[
                'required'=>'Category name is required',
                'is_unique'=>'Category name has already been assigned, try another name'
            ]
        ],       
    ];

regards

Newbie here in CodeIgniter World !! Please appologize if any nonsense from me.  

environment: Windows 10, XAMPP 3.3.0, VS Code, SQLyog Community Ed.