Laravel 5.5 duplicate insert to database once submit

can someone help me...my laravel code insert twice to database when I click submit, it store double data to database, here is my code : controller

public function store(Request $request)
{   
    $this ->validate($request,[
        'nim' => 'required|max:8',
        'nama' => 'required|max:30',
        'alamat' => 'required|max:100',
        'jenis_kelamin' => 'required|max:9',
        'no_tlp' => 'required|regex:/[0-9]{12}/',
        'tempat' => 'required',
        'tanggal' => 'required',
        'id_jurusan' => 'required'
    ]);
    $simpan = new Mahasiswa([
        'nim' => $request->get('nim'),
        'nama' => $request->get('nama'),
        'alamat' => $request->get('alamat'),
        'jenis_kelamin' => $request->get('jenis_kelamin'),
        'no_tlp' => $request->get('no_tlp'),
        'tempat' => $request->get('tempat'),
        'tanggal' => $request->get('tanggal'),
        'id_jurusan' => $request->get('id_jurusan')
    ]);
    $simpan->save();

    if (Mahasiswa::create($request->all())) {
        $request->session()->flash('status', 'success');
        $request->session()->flash('pesan', 'Data Berhasil Disimpan');
    }else{
        $request->session()->flash('status', 'danger');
        $request->session()->flash('pesan', 'Data gagal Disimpan!!');
    }

    return redirect('/Mahasiswa/create');
}

and this my model code :

class Mahasiswa extends Model
{
    protected $fillable = ['nim','nama','alamat','jenis_kelamin','no_tlp','tempat','tanggal','id_jurusan'];
    protected $table = 'mahasiswa';
}

Thanks before...

via Chebli Mohamed