Understand database uniqueness
Currently you can "Include in Unique Index" for tables you create inside LS and LS handles the error on misentry of a duplicate. However, LS fails to handle existing columns in an existing database that have a uniqueness index on them properly. First, it fails to mark them with "Include in Unique Index" on the properties screen. Second, on misentry of duplicate value, the database errors rather than having a LS validation error. Third, LS fails to show you the "Include in Unique Index" option so you could adding uniqueness to existing column that did not have one before.
Roger Hendriks commented
Multi column option also would be great