Rated 3 out of 5 stars

Not quite what I was looking for -- I need something that will automatically show a tab if it is already open, rather then just give me a button to close duplicates.

Edit, for clarification: When I type a URL into the location bar of a tab (and press enter), click on a link for that URL, or open a link from the desktop, and there is already another tab open with that URL, I want firefox to automatically activate that other tab, instead of creating a new one.

The closest that Firefox has gotten to this in its default behavior is that it will show "Switch to Tab" as one option in the list from the awesome bar when typing in a URL that is already open. This is great as a reminder, but it's not automatic, and it doesn't help when clicking on links.

I've heard that this is possible in chrome, but I've never used that. I'd originally checked out deduplicate-tabs because I thought it might have this functionality.

This review is for a previous version of the add-on (0.12). 

I don't understand

That's default behavior of Firefox since at least version 3, isn't it?