Request validation with laravel 5

I have problem when trying validate an input in the form, catch an error of "unique". Although I am modifying and not inserting a new record... This eror is The element is already in use codigotiporiesgo

Look at the code: FORM:

<div class="form-group" id='test'>
 {!!Form::label('codigoTipoRiesgo', 'C&oacute;digo', array('class' => 'col-sm-2 control-label')) !!}
      <div class="col-sm-10">
        <div class="input-group">
          <span class="input-group-addon">
            <i class="fa fa-barcode"></i>
          </span>
          <input type="hidden" id="token" value=""/>
          {!!Form::text('codigoTipoRiesgo',null,['class'=>'form-control','placeholder'=>'Ingresa el código del tipo de riesgo'])!!}
          {!!Form::hidden('idTipoRiesgo', null, array('id' => 'idTipoRiesgo')) !!}
        </div>
      </div>
 </div>

and REQUEST:

"codigoTipoRiesgo" => "required|string|max:20|unique:tiporiesgo,codigoTipoRiesgo,".$this->get('idTipoRiesgo') .",idTipoRiesgo",
"nombreTipoRiesgo" => "required|string|max:80",
"origenTipoRiesgo" => "required|string",
"ClasificacionRiesgo_idClasificacionRiesgo" => "required|numeric"

via Chebli Mohamed