listbox item values skip same values

I have a listbox that hte user populates with items.  Then a foreach loop is fired to insert into a table for each item in the listbox.  To avoid duplicate data, I want to check to see if the listboxitem's value is equal to another listbox item and only insert one of them.  How could I add this somewhere in my code to prevent duplicates in my table? 

 

Or maybe I can prevent duplicate items from being added to it?  Heres the code to add things to the listbox:

 protected void btnAttend_Click(object sender, EventArgs e)
{
foreach (ListItem li in lbResidents.Items)
{
if (li.Selected)
{


lbAttended.Items.Add(li);
}
}
} ...