Often, a tournament with final rounds has no resit options: a defeat results irrevocably in elimination of the losing team.

In contrast, a double-elimination tournament is one where a competitor is excluded from the final only after losing two games.

With mytournament.info you can easily set up a tournament with double elimination.

In the category settings, select Double elimination if you do not want to play a group phase beforehand:

If a group phase is desirable, do choose Groups, double elimination:

Participants will be divided (possibly after a group phase) into two groups after the first elimination round, the winner's group and the loser's group.

If the number of participants is not equal to a power of two (8, 16, 32, etc.), there will be ‘byes’ in the first elimination round.
A ‘bye’ means for a team to reach the next round of competition without playing a match.

The number of matches in a double-elimination tournament is one or two less than twice the number of participating teams.

If a team that has not lost any matches in the previous elimination rounds but loses in the final, an extra (decisive) final match can be played.
The winner of the (extra) final is the only team that has not lost more than one match.

The schedule of simple tournament with 2 groups, 4 teams and double elimination is shown below: