When defining the Storyboard as part of the resources of an element of the template VisualTree

<ControlTemplate x:Key="t" TargetType="Control">

<Border x:Name="root" Background="Red">

<Border.Resources>

<Storyboard x:Key="intro">

<ColorAnimation To="Blue" Duration="0:0:0.5" Storyboard.TargetName="root" Storyboard.TargetProperty="Background.Color"/>

</Storyboard>

</Border.Resources>

<i:Interaction.Triggers>

<i:EventTrigger EventName="MouseEnter">

<ei:ControlStoryboardAction Storyboard="{StaticResource intro}"/>

</i:EventTrigger>

<ei:StoryboardCompletedTrigger Storyboard="{StaticResource intro}">

<ei:ChangePropertyAction TargetName="root" PropertyName="Width" Value="150"/>

</ei:StoryboardCompletedTrigger>

</i:Interaction.Triggers>

</Border>

</ControlTemplate>

all instances of the template share the same Storyboard and trigger the Completed event for all controls.

In WPF each control instance has its own copy of the Storyboard and the Completed event triggers only for the corresponding instance.