WPF: Can use StaticResource only once

I have a Static resource defined in my WPF Windows XAML:

<Window.Resources>
    <Image x:Key="MyImage" Source="../Icons/img.png" Width="16" Height="16" Stretch="None" />
</Window.Resources>

I want to use it two times:

<Grid>
    <Button Content="{StaticResource MyImage}"  RenderOptions.BitmapScalingMode="NearestNeighbor" RenderOptions.EdgeMode="Aliased" />
</Grid>

...

<Grid>
    <Button Content="{StaticResource MyImage}"  RenderOptions.BitmapScalingMode="NearestNeighbor" RenderOptions.EdgeMode="Aliased" />
</Grid>

But it gets displayed as button image only once. On the last button. The first button has no image.

When I remove the second button then it works for the first one. How to use the StaticResource multiple times? The Visual Studio GUI Designer displays the images on both buttons.