Disable storage of originals on local drive

When you add photos to the Lightroom desktop app, the first thing that happens is that Lightroom copies the images being added into its own space, and it's from that location that the images are then uploaded to the cloud. The user cannot prevent that inital copy being made.

So, if you are adding photos from an existing hard drive location, then yes there is an initial duplication. As you have the "Store a local copy" option unchecked, that means that the copy that Lightroom makes becomes eligible for deletion once the cloud upload has completed (Lightroom subsequently ignores your original hard drive copies). However, that deletion is subject to some apparently complex rules, which can mean that the Lightroom copies hang around for longer than perhaps desired. Hopefully, as Adobe are aware of this issue, they will change the rules in due course to allow for a more timely deletion. In the meantime, provided you are certain that the files have all been safely uploaded, you can if you wish delete the Lightroom local copy. Note, however, that there will always be a certain amount of churn in that local copy location as editing images in the Lightroom desktop app will result in a copy being downloaded into that local location if it's not currently there, and again it will then become subject to those complex rules before it is deleted again.