Delphi items compare between 2 listboxes

Question

I want to know how to compare items between listboxes. On my main form there are two listboxes. I want to add item from 1st to 2nd with a click event but when use it, the same item will multiply on 2nd listbox. Any idea to solve "the file already exists" ?

procedure TForm1.Button1Click(Sender: TObject);
var
  i: integer;
begin
  for i := ListBox1.Items.Count - 1 downto 0 do
    if ListBox1.Selected[i] then                       
      ListBox2.Items.Add(ListBox1.Items.Strings[i]);
end;

Answer 1

If you got a single-select listbox1:

if Listbox2.Items.IndexOf(Listbox1.Items[Listbox1.ItemIndex]) = -1 then
begin
  // Doesn't exist yet. Safe to add
end;

For multi-select (which your code seems to imply):

for i := 0 to ListBox1.Items.Count - 1 do
  if (ListBox1.Selected[i] and (ListBox2.Items.IndexOf(ListBox1.Items[i]) = -1) then                       
    ListBox2.Items.Add(ListBox1.Items[i]);

The latter will work for single-select too, I think.