Unless you are backing up to CD/DVD, Deja Vu should always perform an incremental backup, meaning that only new and/or changed files should be copied each time. There is no setting for this.
File sizes and modification times are compared between the source and destination to determine which files need to be copied.
If you are backing up over a network, verify that the system clocks are set correctly on both computers.
Also, keep in mind that the system may be making changes to certain files even if you yourself have not made any changes to them.