Error adding item with same key when submit

@model namespace.ViewModels.SampleViewModel
...omitted...
@using(Html.BeginForm("Index", "Controller Name", FormMethod.Post))
{
    ...omitted...

    <td><input type="submit" value="Registration"/>/td>

}

// Initial View
[HttpGet]
public ActionResult Index (stringdType)
{
    varViewModel=new Sample ViewModel();

    … (all set in ViewModel) …

    return View (ViewModel);
｝

// registration
[HttpPost]
public ActionResult Index (sample ViewModel vm)
{
    ...omitted...
}