Not a bad addon but could use a lot more functionality. For starters, it needs a whitelist for sites that are okay to duplicate, which is often necessary. Also, a simple "Close tab automatically" function really isn't the best way to handle a duplicate inadvertently being opened. For example, say you forget you've already got Google open and you type a search in the address bar (which I have set to auto open in a new tab). Rather than the new tab opening with the search I just typed, it switches to the old tab, with an old search in it, which is at first pretty disconcerting. In this case it makes more sense to close the old tab, not the new one, but again, maybe I actually do want them both open. There needs to be a white list. Did I say there needs to be a white list? There needs to be a white list.
RESPONSE TO DEVELOPER'S COMMENT: If you think I gave it two stars just for missing one feature, you didn't even bother to read the whole review. Two stars stands I now I don't even want to bother seeing if it's improved. You need to learn what reviews are for.