Model error and re-retrieving combo-box items

On loading up a form, I retrieve a list of items to put in a combo-box. Now lets say during the post, there is an error. I have to re-write the code to get and build back my viewmodel again?

For eg.

    public ActionResult Index()
    {
        var vModel = GetViewModel();
        return View(vModel);
    }

    [HttpPost]
    public ActionResult Index(SomeModel model)
    {
        if (ModelState.IsValid)
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        }
        else
        {
            //Why do I have to write monkey-code here?
            var vModel = GetViewModel();
            return View(vModel);
        }
    } 

Am I going to have to do it like this example where I have a method to build my viewmodel for both actions?

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Why not just return Index(); instead?