title: Data Error 26 with DataTemplate and ItemTemplate
body: <p>I have a TabControl with a single specific tab and a collection bound to a collection of VMs, using a different user control. To do this I use a CompositeCollection and DataTemplates defined in the control's resources, selecting correct user control based on the VM type (acting as ContentTemplate).</p>
  
  <p>I also set an ItemTemplate to define the tab item's name with binding, but it's not defined in the resource as I guess would conflict with the "ContentTemplate" ones.</p>
  
  <p>It works fine, but I see the following error traced:</p>
  
  <blockquote>
    <p>System.Windows.Data Error: 26 : ItemTemplate and ItemTemplateSelector are ignored for items already of the ItemsControl's container type; Type='TabItem'</p>
  </blockquote>
  
  <p>It looks like there's some conflict between ContentTemplate and ItemTemplate, but I don't know how to fix it?</p>
  
  <p>Code is the following:</p>
  
  <pre><code>&lt;TabControl HorizontalAlignment="Left" Height="300" Width="500"&gt;
      &lt;TabControl.Resources&gt;
          &lt;CollectionViewSource x:Key="personCollection" Source="{Binding Persons}" /&gt;
          &lt;DataTemplate DataType="{x:Type viewModel:Main}"&gt;
              &lt;local:MainView /&gt;
          &lt;/DataTemplate&gt;
          &lt;DataTemplate DataType="{x:Type viewModel:Person}"&gt;
              &lt;local:PersonView /&gt;
          &lt;/DataTemplate&gt;
      &lt;/TabControl.Resources&gt;
      &lt;TabControl.ItemsSource&gt;
          &lt;CompositeCollection&gt;
              &lt;TabItem Header="General" Content="{Binding }"/&gt;
              &lt;CollectionContainer Collection="{Binding Source={StaticResource personCollection}}" /&gt;
          &lt;/CompositeCollection&gt;
      &lt;/TabControl.ItemsSource&gt;
      &lt;TabControl.ItemTemplate&gt;
          &lt;DataTemplate DataType="viewModel:Person"&gt;
              &lt;TextBlock Text="{Binding FirstName}" /&gt;
          &lt;/DataTemplate&gt;
      &lt;/TabControl.ItemTemplate&gt;
  &lt;/TabControl&gt;
  </code></pre>