Functional extension with many extra options. It checks the container of each tab, so there's no risk of closing tabs with the same URL, but with different credentials (be careful, though, that if a site manages states using cookies, local storage or anything else that doesn't change the URL, the extension will consider the tabs as duplicates and close them.) It indicates how many duplicate tabs are there in a window. It has an option to keep the oldest tab, instead of the newest. If offers three optional hotkeys to remove duplicate tabs.

Useful as alternative to "Don't Open Duplicate tabs"

I worked with author on this addon after trying every other addon that mention duplicate tabs. This addon two things: 1. shows count of duplicates tab with a badge on the icon, and 2. delete the duplicates tab when you click the icon.