DoubleEditBox Firing ValueChanged Twice When Bound To Float

I've got a DoubleEditBox in a ListBox's ItemTemplate, like this:

                    <ListBox ItemsSource="{Binding Offsets}" >
                        <ListBox.ItemTemplate>
                            <DataTemplate>
                                <actiedit:DoubleEditBox Format="F2" Minimum="-1" Maximum="1" SmallChange=".01" Value="{Binding Path=Lengthwise}" ValueChanged="DoubleEditBox_ValueChanged" />
                            </DataTemplate>
                        </ListBox.ItemTemplate>
                    </ListBox>

The Offsets property is just:

        public List<MyObj> Offsets
        {
            get { return _offsets; }
            set { _offsets = value; RaisePropertyChanged(); }
        }
        private List<MyObj> _offsets = new List<MyObj>();

        public class MyObj
        {
            public float Lengthwise { get; set; }
        }

This works perfectly - however, every time a Lengthwise value changes, the ValueChanged event fires *twice*.  If I change Lengthwise to double, then it works the same, but the event fires once.

Why would binding to float cause every event to be fired a duplicate time, but binding to double not (and is there a way to bind to float while avoiding these duplicate events?)