They have some major differences. Enough so that I first tried Duplicati and ran into corruption issues so frequently that I sought out an alternative and luckily found Duplicacy.

Duplicacy has been stable for years now and I gladly pay the commercial license. It seemed like Duplicacy constructs a giant DB of all the files and manages everything that way, whereas Duplicacy's approach is much simpler and is less prone to corruption. The large DB approach seems to fail when the backup set contains a large number of files that many users manage.