I tried a duplicate tabs extension that was way simpler than this that I liked at first. It could close duplicate tabs automatically in the background without interruptions, but it stopped working at some point. I tried this one next and it seems to be very robust. I wasn't sure that I needed all the bells and whistles at first but they have turned out to be quite useful after all, such as the URL format filters and the whitelist.

If I could add a suggestion, it would be super helpful to be able to whitelist a new tab. Not sure if this is possible. I tried entering a pair of empty double quotes into the whitelist field "" to see if that would work since I noticed that wildcards are also supported. Kind of a hackerly solution but I took a swing at it. Still closes them though. 😛