Steam Owned

Checks if you already have a game in Steam, Epic, GOG library, itch.io, Fanatical

Works on many websites. You can run it manually via the extension icon on any website. https://alexa.weezeewig.com/SteamOwned/SupportedWebsites.html

This extension only looks at your list of games and wishlists. This information is kept on your computer and not sent anywhere. You can see which sites it has access to in "settings" > "extensions" > "details"

To use...

Go to one of the supported websites.

Click on the "library" icon on the top right of the browser.