The following code shows how to use the property Duplicates to avoid duplicate items in string lists:
var
sl : TStringList;
begin
sl := TStringList.Create;
with sl do
begin
Sorted := True;
Duplicates := dupIgnore;
//
// add your items here
// all the duplicate items
// will be ignored
//
Add(Input1.text);
end;
//
// now you can assign the
// string list with unique
// items to listbox1 (example)
//
ListBox1.Items.Assign(sl);
sl.Free;
end;