Just to butt in. I do hope that you are at least going to put in the corner of the UI
“Powered by Duplicati” with the duplicati logo, Not just use a small text footnote hidden away in the about section.
You can do what you like as long as it’s in the licence, but do keep in mind that this is on LGPL for your convenience, and you may want to make sure you make it Known what tech is powering your rebrand, so if you ever do give up on your project and don’t keep up with updates, People using your tool will at least be able to get up and running on the main core and get important updates.
You should also know that you should forgive the hostility on this topic, but the OpenSouce community are quite touchy on this because of scammers that rebrand and sell OpenSoftware without meaningful modification beyond rebranding.
Do it right, Do it with class and if you are selling a modification, explain that the core technology is free, and sell your product on what you have done extra that is worth what you are charging. Also keep in mind that you are getting a lot of groundwork free, so consider having a voluntary kickback, even if it is 5% of ernings, and/or submitting any inproved or bug fixed code back to the dev team.
Again, Do it right, Do it with class.
Then again, I’m british. So this may just be my british ideals of fairness, politeness and accountability showing.