Custom Error Messages after submitting data Laravel

I already asked this question but there are a few things different this time. Last time the problem was fixed pretty well so now i just need a hand to tell me how to change the code so it works properly.

The thing i have changed is that i have implemented a way to successfully lend more then one book at once. So now i have an array which works perfectly.

So this is my View imagine this code 3 times one for every book you want to lend:

<div class="form-group row">
     <label for="serialnumber" class="col-md-4 col-form-label text-md-right"></label>
     <div class="col-md-6">
          <input id="serialnumber" type="text" class="form-control" name="serialnumber[]" value="" required   @if (Session::has('autofocus')) autofocus @endif>
          @if ($errors->any())
               <div class="alert alert-danger">The book with this serialnumber is already lend by antoher person
                    <ul>

                    </ul>
                </div>
          @endif
      </div>
</div>

This is my Controller Code now:

public function store(BookRequest $request)
{

      //if( !Book::find($request->get('serialnumber'))->exists() ) {
        $this->middleware('guest');

        request()->validate([
          'serialnumber' => 'required',
          'ma_id' => 'required'
        ]);

        $requestData = $request->all();

$data = [
    [
       'serialnumber' => $requestData['serialnumber'][0],
       'comment'    => $requestData['comment'],
       'ma_id'        => $requestData['ma_id'],


    ],
    [
       'serialnumber' => $requestData['serialnumber'][1],
       'comment'    => $requestData['comment'],
       'ma_id'        => $requestData['ma_id'],

    ],

    [
       'serialnumber' => $requestData['serialnumber'][2],
       'comment'    => $requestData['comment'],
       'ma_id'        => $requestData['ma_id'],

    ]
];

        Book::insert($data);

        return redirect()->route('borrow.index')
                         ->with('success','Successfully lend the book');

    }

And the last is my Request.php page:

<?php

namespace App\Http\Requests;

use Illuminate\Foundation\Http\FormRequest;

class BookRequest extends FormRequest
{
    public function authorize()
    {
         return true;
    }

    public function rules()
    {
        return [
          'serialnumber[0]'  => 'required|unique:borrowed,serialnumber,null',
          'serialnumber[1]'  => 'required|unique:borrowed,serialnumber,null',
          'serialnumber[2]'  => 'required|unique:borrowed,serialnumber,null',
          'ma_id'      => 'required',

        ];
    }

    public function messages()
    {
        return [
            'serialnumber' => 'Seems like you have added the same book more than once!',
       ];
    }
}

And this is my error message which i got after i tried to lend a book which is already lend by another person. Before i implemented the array thing this code worked perfect. Another question that i have is how could i implement a way which shows an error message which says "Sorry but this book is currently not in our database please press the info button and get some administraive help" so that basically an error message appears when the book is not in our database we have a lot of books so it is possible that we forget to scan one. Every help is much appreciated!!

EDIT: Forgot the error message

htmlspecialchars() expects parameter 1 to be string, array given

via Chebli Mohamed