You can add a style to ListBox.ItemContainerStyle, and add an EventSetter there:
<ListBox>
....
<ListBox.ItemContainerStyle>
<Style TargetType="{x:Type ListBoxItem}" BasedOn="{StaticResource {x:Type ListBoxItem}}">
<EventSetter Event="MouseDoubleClick" Handler="ListBoxItem_MouseDoubleClick"/>
</Style>
</ListBox.ItemContainerStyle>
</ListBox>
ListBoxItem_MouseDoubleClick is a method in your code behind with the correct signature for MouseDoubleClick.
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