Validating uniqueness of JSON column with Form Validation

Hello everyone, I'm relatively new to Laravel and PHP in general, and right now I need some help with validation. I hope someone can help me.

My project uses a coordinates column that is a json type. For clarification, it doesn't use real world coordinates, but rather an array like this:

{
   "atmosphere": {
      "x": 0,
      "y": 0
   },
   "ground": {
      "x": 0,
      "y": 0
   }
}

These should be unique coordinates so I would like to know if and how it's possible to validate this using Laravel's Form Validation.

I have this form request:

public function rules()
    {
        $id = $this->segment(3);

        return [
            'id' => ['required','integer',Rule::unique('locations')->ignore($id)],
            'market_id' => 'required|integer',
            'coordinates' => ['required','array',Rule::unique('locations')->ignore($id)],
            'name' => 'nullable|string',
            'owner' => 'nullable|string',
        ];
    }

The rest of the validation works fine, except for the coordinates field, since I can continue creating models with the same coordinates over and over.

I tried using the rule json instead of array, and casting it with this function:

protected function prepareForValidation()
{
    $this->merge([
        'coordinates' => json_encode($this->coordinates),
    ]);
}

But it still doesn't do the validation, and instead saves the entry with backslashes, rather than a clean array. My guess is that I need a custom Rule but I'm not exactly sure how to do it.

I'll appreciate any insight you guys can give me. Thanks in advance