A tie at the same total, with your bet returned.
When you and the dealer have the same total, neither side wins and your bet is returned. Example: you have 19 and the dealer also has 19, so it's a push. In Double Up, when the dealer stands on 16, anything other than 21 pushes (the DU portion comes back too). But on a tie at the same total (19 against 19, say), the default setting makes the DU portion lose.