UpdateModel Exception - asp.net-mvc-3

Question

I'm trying to assign some values from FormCollection in my controller using UpdateModel.
Controller looks like:
public ActionResult EditValues(int id, FormCollection collection)
{
NamedClass picture = PictureProvider.GetById(id);
try
{
if(ModelState.IsValid)
{
UpdateModel(picture, collection);
}
}
catch {}
}
and at string UpdateModel(picture, collection); I've got an ArgumentException that says "Element with the same key has already been added" and nothing more..
"Collection" has same fields as "picture".. I'm not trying to add anything to "collection" that may cause such exception.. Tried to assign value to picture's field manually - OK.
Has anybody seen such trick? Thank you in advance!
UPD: May be StackTrace will be helpful..
System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
System.Web.Mvc.ModelBindingContext.get_PropertyMetadata()
System.Web.Mvc.DefaultModelBinder.BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor)
System.Web.Mvc.DefaultModelBinder.BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext)
System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model)
System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
System.Web.Mvc.Controller.TryUpdateModel[TModel](TModel model, String prefix, String[] includeProperties, String[] excludeProperties, IValueProvider valueProvider)
System.Web.Mvc.Controller.UpdateModel[TModel](TModel model, String prefix, String[] includeProperties, String[] excludeProperties, IValueProvider valueProvider)
System.Web.Mvc.Controller.UpdateModel[TModel](TModel model, IValueProvider valueProvider)
Namedclass is:
public class NamedClass
{
[Key]
public virtual int id {get; set;}
public virtual string username {get; set;}
}
and the View that provides values is:
#model project.Models.NamedClass
#using (Html.BeginForm("EditValues", "PicController", FormMethod.Post))
{
#Html.HiddenFor(m=>m.id)
<div class="display-label">#Html.LabelFor(model=>model.username)</div>
<div class="display-field">
#Html.EditorFor(model => model.username)
</div>
<input type="submit" value="save" />
}

Answer 1

Why can't you just do it like this.
public ActionResult EditValues(NamedClass model)
{
if(ModelState.IsValid)
{
.. save to db
}
return View();
}