Also, how do you see this as being different from Duplicati and Arq?
The main use case supported by Duplicacy but not any others including Duplicati and Arq is backing up multiple clients to the same storage while still taking advantage of cross-client deduplication. This is because Duplicacy saves each chunk as an individual file using its hash as the file name (as opposed to using a chunk database to maintain the mapping between chunks and actual files), so no locking is required with multiple clients. Another implication is that the lock-free implementation is actually simpler without the chunk database and thus less error-prone.
one of our users wrote a long post (https://duplicacy.com/issue?id=5651874166341632) comparing Duplicacy with other tools including Arq, based on his experience. I also added a comment to that thread comparing Duplicacy with Arq based on my read of their documentation.