title: aspnet web forms An item with the same key has already been added
body: <p>I am using asp net 4.5.2 with model binding; I have a problem that whenever I try to use model binding on DropDownList It raises the error <code>An item with the same key has already been added</code> but if I change it to SQLDataSource the error goes away.
  Thanks in advance</p>
  
  <hr>
  
  <p>Update</p>
  
  <p>Sample Code</p>
  
  <pre><code>&lt;asp:ListView runat="server" ID="lvChairItem" DataKeyNames="ChairItemId" InsertItemPosition="FirstItem" OnItemCanceling="lv_ItemCanceling" OnItemUpdated="lv_ItemUpdated" OnItemEditing="lv_ItemEditing" ClientIDMode="AutoID" OnCallingDataMethods="ChairItem_CallingDataMethods" SelectMethod="GetByChairId" InsertMethod="InsertItem" UpdateMethod="UpdateItem" DeleteMethod="DeleteItem" ItemType="App.Model.ChairItem"&gt;
  .............
  &lt;InsertItemTemplate&gt;
      &lt;tr&gt;
          ........
          &lt;td&gt;
              &lt;asp:DropDownList ID="ddlRoomId" CssClass="" DataValueField="Value" DataTextField="Text" AppendDataBoundItems="true" Text='&lt;%# BindItem.RoomId %&gt;' Width="100%" OnCallingDataMethods="Room_CallingDataMethods" SelectMethod="GetAllForDDL" runat="server" EnableViewState="true"&gt;
                  &lt;asp:ListItem Value=""&gt;--Select--&lt;/asp:ListItem&gt;
              &lt;/asp:DropDownList&gt;
          &lt;/td&gt;
          ........
      &lt;/tr&gt;
  &lt;/InsertItemTemplate&gt;
  .....................
  &lt;/asp:ListView&gt;
  </code></pre>