They can't deduplicate properly encrypted data, that will have to happen client-side. So while they can deduplicate your data (if you have a copy of a picture in multiple places or maybe even a game installed on multiple machines), it won't work across users.

In that case I’d also expect them to skip uploading the data, but everything is uploaded.

they'd probably charge you the same anyway, even if THEY get it at basically free ;)