https://restic.net/ (This can use rclone as backend for backups, but can't sync and has the worst file reduction compared to borg or duplicati.
I know there have been other compression requests, but combining with deduplication and seeing how borg and duplicati do may assist. They produce amazing results, but unfortunately no sync and less backends.
De-duplication in the way that assume you mean does not exist currently, however you may find --copy-dest and --compare-dest useful even if they aren't quite the overarching de-duplication you are probably thinking about. https://rclone.org/docs/#compare-dest-dir
You can make formal feature requests here (so they aren't just forgotten in a forum thread that grows old):
But you should absolutely see if there already exists an issue for it an if so - upvote that rather than creating a duplicate issue about it...
Of course you can use any third-party deduper software via a mount if you wish... but that's more about cleaning up files after the transfer than saving space in transferring them.