Main.xaml

<Grid
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Grid.Resources>
    <ControlTemplate x:Key="Spinning_Widget">
      <ControlTemplate.Triggers>
        <EventTrigger RoutedEvent="Loaded">
          <BeginStoryboard>
            <BeginStoryboard.Storyboard>
              <Storyboard>
                <DoubleAnimationUsingKeyFrames Storyboard.TargetName="Root" Storyboard.TargetProperty="(UIElement.RenderTransform).(RotateTransform.Angle)" RepeatBehavior="Forever">
                  <LinearDoubleKeyFrame KeyTime="0" Value="0" />
                  <LinearDoubleKeyFrame KeyTime="0:0:2" Value="360" />
                </DoubleAnimationUsingKeyFrames>
              </Storyboard>
            </BeginStoryboard.Storyboard>
          </BeginStoryboard>
        </EventTrigger>
      </ControlTemplate.Triggers>
      <Rectangle x:Name="Root" Fill="Red" Width="50" Height="50" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
          <RotateTransform Angle="0" />
        </Rectangle.RenderTransform>
      </Rectangle>
    </ControlTemplate>
    <ControlTemplate x:Key="Common_Widget">
      <Grid Width="100" Height="100" Background="Gray">
        <!-- Is there any way to make it so we render a shared single instance of Spinning_Widget here - I.e. only render 1 control and duplicate it multiple times? -->
        <Control Template="{StaticResource Spinning_Widget}" HorizontalAlignment="Center" VerticalAlignment="Center" />
      </Grid>
    </ControlTemplate>
  </Grid.Resources>
  <StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
    <Control Template="{StaticResource Common_Widget}"/>
    <Control Template="{StaticResource Common_Widget}"/>
  </StackPanel>
</Grid>