It's amazing that this is a product. In an earlier job we used to ask how to do this (discovery phase of deduplication) in the first interview screening. Once you have that list, it's straightforward to make a few syscalls that make it happen.
If you're referencing Fetch for mac (pre macos) or Transit by Panic (later renamed to Transmit), I was a fan (and I've been a terminal fanboy since the early 80s). Both of them started as FTP wrappers, and both provided convenience the cli lacked.
As I recall, there are nuances around certain types of files that require expert attention, such as links, files inside frameworks/packages, and unmaterialized iCloud Drive files.
