Using an attached property to populate the SelectedItems of the control initially selects only the first item.

A sample project (ListBoxSelectedItems - Example) of the used approach can be found in this blog post:
http://blogs.telerik.com/xamlteam/posts/13-01-14/extending-the-functionality-of-radcontrols-with-attached-properties

Implement IsSynchronizedWithCurrentItem property in order to disconnect the connection with CollectionViewSource.

The SelectionMode must be Multiple.

The SelectionMode must be Multiple.

When the control is initially Collapsed setting the SelectedIndex does not update the SelectedItem property of the control and vice versa.