If you have opened too many tabs in your browser, instead of opening same website again unknowingly you opened that website in a new tab, this way you end with lot of duplicate tabs in your browser which intentionally you don’t want.  Close duplicates extension for Opera shows button besides search bar with a badge of duplicate tabs it discovered.With a single click at that button all duplicate tabs will be closed.

Close duplicates extension removes all duplicate tabs in Opera

Close duplicates Opera extension