feature request: ability to find all duplicate tabs - perhaps even prune duplicates!
There's plenty of add-ons that already are specialized an exactly that feature. I personally don't feel like it's a good fit to cram a feature like that into Vimium, since it's not even focused on navigating the browser with the keyboard, so to me it seems kinda off-topic.
See also Vimium's design principles.
deduping tabs is a form of tab administation. also say on a company computer, you cannot simply add extra plugins. requiring a process of assesment etc. so id rather see more features in a plugin i trust and use for all my existing tab jumping and link clicking.
You can use Shift + T to find duplicate Tabs of specific tab by searching for the name of the tab and then close them manually (prune). If you are unsure if a tab already exists before opening a new tab again you can simply Shift + T first and then open it again if it isnt open already.