There are several users on one PC - user1, user2... User2 has a scheduled daily backup. Will Deja Dup make user2's daily incremental backup, while user1 is logged in and using the PC?
Déjà Dup is really just a clever graphical wrapper around the command line backup tool duplicity. You can read more about it on its homepage, but basically, it uses rsync to generate incremental, encrypted backup volumes.
Déjà Dup does not use cron or similar schedulers. Rather, it starts a program deja-dup-monitor when you log into your session. This keeps track of when you last successfully backed up and will wait until the next scheduled backup.
It determines exactly when the next scheduled backup is largely on its own, based on the user's preferences about whether to backup once a day, once a week, etc.
Why Not Cron?
One disadvantage is that a backup can not be started while a user is not logged in. The primary use case for Déjà Dup is backing up user data, so this is not a large concern, as user data is unlikely to change while the user are not logged in.