Laravel Excel - How to check for duplicates in import file

I am importing new users from an excel file with Laravel-Excel.

I would like to know if it's possible to throw an error if there is two users with a same email in the import file, while keeping the implementation of the current interfaces.

My UsersImport class is declared like so:

class UsersImport implements ToModel, 
 WithHeadingRow,
 WithValidation, 
 WithBatchInserts, 
 WithChunkReading
{
    private $user;
    private $company;

    public function __construct($user, $company)
    {
        $this->user = $user;
        $this->company = $company;
    }

    public function model(array $row)
    {
        if(!isset($this->company)) return;

        if ($this->user->contains('email', $row['email'])) {
            $this->throwNotification($row['email']." est déjà utilisé.");
            return;
        }

        Mail::to($row['email'])->send(new NewUser($this->company->name));

        $employee = new \App\Models\User($row + [
            'company_id' => $this->company->id
         ]);

        return $employee;
    }

    public function rules(): array
    {
        return [
            'firstname' => 'required|string',
            'lastname' => 'required|string',
            '*.email' => ['required', 'email', 'unique|users:email'],
            'phone' => 'nullable|max:15',
        ];
    }

    public function batchSize(): int
    {
        return 250;
    }

    public function chunkSize(): int
    {
        return 250;
    }
}