(This is not a multi-select problem.)
Obviously you've got two list-boxes and want to add one line from the first to the second on double-click.
You need to add that line to the recordSource for the second list-box, i.e. to the table containing data for the second list-box. After that you simply do
Me.ListBox2.Requery
And the item is also shown in the 2nd box.
Hope it solves your problem.
Mic