You may decide to change your Defensive Scheme (at Team->Defense - Defensive Scheme) from a 4-3 to a 3-4 or vice versa. Only problem is the game does not change your playbook. So if you change from a 4-3 to a 3-4 you will have 2 DTs showing a season's worth of stats as though they both started all the games.

Prevent this by always changing your playbook (Team->Playbook - Folder: Defense Balanced - Load Defensive Playbook) to match the Defensive Scheme.