Depending on what you want to compare, you can use Text or Tag (tag being the better choice).
var tags = new HashSet<object>();
var duplicates = new List<ListViewItem>();
foreach(ListViewItem item in listView.Items)
{
if(!tags.Add(item.Tag)
duplicates.Add(item);
}
foreach(ListViewItem item in duplicates) {
if (listView.Items.Contains(item)) {
listView.Items.Remove(item);
}
}