At times we do open two tabs accidentally by clicking the same URL in a page, with so many tabs being open finding a duplicate tab is in fact difficult task, Deduplicate-Tabs add-on for Firefox automatically detects duplicate tabs opened in your browser and allows to remove them with a single click on its icon on Add-on Bar in Firefox.
Download and install the add-on for Firefox browser (no restart required). From now on if you open duplicate pages in Firefox unknowingly, this add-on detects them and shows “deDup” icon on the Add-on bar. Click on deDup icon and duplicate tabs will be removed.
DeDup icon only appears when duplicate tabs are opened. Are you looking for similar kind of extension for Chrome? OnlyOne does the same job for you.