Why Mac App Store shows open button below the macOS sierra installer icon in the product page? Before when I had macOS sierra installed on my mac and when I used to open macOS sierra in the MacApp store there used to be an option to reinstall/download the installer but now I am seeing an open button below the macOS icon after updating to the latest stable build of macOS sierra
Check your /Applications folder for the 'Install macOS Sierra" file. It is usually deleted at the conclusion of the OS installation but you may have still it and the AppStore is seeing it.