Update multiple records at once in asp.net mvc

Your first problem is that your use of a foreach loop is generating duplicate name attributes, which will not bind to a collection, and as a result the BillLists parameter will always be an empty collection (its also generating duplicate id attributes which is invalid html). You need to use a for loop or a custom EditorTemplate for typeof BillCheck. Using a for loop, your view need to be

using (Html.BeginForm("MakeDue", "Home"))
{
  @Html.ValidationSummary(true)
  @for(int i = 0; i < Model.DueList.Count; i++)
  {
    
    <tr>
      <td>
        @Html.HiddenFor(m => m.DueList[i].id)
        @Html.DisplayFor(m => m.DueList[i].flat)</td>
      <td>@Html.DisplayFor(m => m.DueList[i].name)</td>
      <td>@Html.TextBoxFor(m => m.DueList[i].due)</td>
    </tr>
  }
  <input type="submit" class="btn btn-success" value="Update" />
}

Note also that the @Html.HiddenFor() helper need to be inside a <td> element in order to be valid html.

The next problem is that the model in the view is not typeof List<BillCheck>, but it does contain a property named DueList, which is typeof List<BillCheck> so your POST method needs to be

public ActionResult MakeDue(YourModel model)

where YourModel is the class name you used to generate the view (i.e. in the @model ??? statement). Then you loop in the controller method need to be

foreach (var BillId in model.DueList)
{
  var getDue = db.BillChecks.Where(p => p.id == BillId.id).FirstOrDefault();
  if (getDue != null) // add this
  {
    getDue.due = BillId.due;
  }
}
db.SaveChanges();

Note also the addition of the if (getDue != null) check.

Side note: Your are checking if (ModelState.IsValid). It is recommended you return the view if ModelState is not valid so that the user can correct any errors.