DupTabX от stapmoshun

Based on duplicate tabs closer. https://addons.mozilla.org/en-US/firefox/addon/duplicate-tabs-closer/
But better.

One new feature that users seek is the ability to delete all duplicate tabs on a per-group basis. Say for example you have five Google.com's open. There is now a button that deletes all but one, so it deletes four of them and leaves the latest one.
Also, weird height was fixed and overall looks much better.

https://github.com/jpfleischer/duplicate-tabs-closer/