Pardon my vague naming, I’m doing this from memory.
Open system settings, go to users and select your user, in there you should find an area to select what apps start when the user logs in. Click the plus button then browse to /Applications and select Duplicati.
If you need more detail, let me know and get on a Mac for some screen shots.
Oh, and you can also consider installing as a service / daemon so backups will happen when the computer is turned on rather than only when the user is logged in.
Thanks for ing my post - if the steps offered there helped you fix you problem please consider clicking the “This reply solves the problem” box so other users can see this topic as solved. Enjoy all your new free time not being spent worrying about backups!