How to know if a specific notification has been sent in laravel

I am building a site where users can buy a plan for whatever estimated time they require. After they buy the plan I created a middleware to check when the plan is almost finished (70% into the plan) then send a notification . Now the issue is I am trying to find out if the Notification in question has already been sent before sending another notification. I used database notification method.

To achieve this I had to create a Notification model to interact with my notification table.

    <?php

    namespace App;

    use Illuminate\Database\Eloquent\Model;

    class Notification extends Model
    {
        protected $table= "notifications"; 
    }

The middle ware in question

 public function handle($request, Closure $next)
    {
       // dd(Carbon::now()->format("Y-m-d"));
       $plan = Booking::where("user_id", Auth::id())
                        ->where("approve", true)
                        ->where("end_date", '>', Carbon::now()->format("Y-m-d"))
                        ->get();
        $i =0;
        $save = [];

        foreach($plan as $plan){

           $save[$i] = (new Plan)->percentageMeter($plan->start_date, $plan->end_date);
           if($save[$i] >= 15){

                $realNotify= Notification::where("notifiable_id", $plan->user_id)
                                    ->get();
                foreach($realNotify as $notify){

                    if($notify->data['plan_id'] == $plan->id && realNotify ==true ){
                        continue;

                    }
                    else{

                        ComNotify::planAboutToExpire(Booking::find($plan->id));

                    }
                }



               continue;

           }

           $i++;
        }
    // dd($save);

    }

one of the major reason I have this problem is that I cant access the "plan_id" object because it does not have its own table column.

Is there a Laravel way to do this? a neat way.

public function toArray($notifiable)
    {
        return [
            "type"=>"plan_about_to_expire",
            "message"=>"Your plan will soon expire on " . $this->plan->end_date,
            "plan_id"=>$this->plan->id,


        ];
    }

My notification toArray method