Opera browser has a feature that lets you know about duplicated opened tabs. It annotates duplicated tabs with a blue bar at the bottom of the tab in the tab bar (whenever you hover over one of them). I'd like to see this feature extended to also inform me if a given link I am hovering over on a page has already been opened in a separate tab. This'd help me navigate and better organize my current workspace.
Use case example
I am researching some complex topic (with a lot of link references from the original page to other pages) which requires me to have several tabs open and reference one another from time to time. Having the requested feature would save me the effort of figuring out which sub-pages are already opened, which in turn would help to get back to the context of browsing the original page.