App Store still shows download option for Mac OS High Sierra? I have already installed macOS High Sierra.

But it still shows download option in the App Store.



  Also, the some of the new features that High Sierra is providing like Automatic stopping of videos with sound in Safari, was already updated in my system before I even clicked the download button for installing macOS High Sierra.
The download button in App Store is there by design, so you can download a backup copy, a copy for another computer, virtual machine, etc.

Safari 11, the version in High Sierra, is available as a update for Sierra and El Capitan, which enables features like autoplay sound blocking. You probably had auto updates enabled if you don't remember installing it.