OnlyOne is a Google Chrome Extension designed specifically for Mac users. If you have just switched from a PC to a Mac, this extension is a must-have. If you love working with many tabs and constantly switch tabs, you probably forget which tab has which website open and tend to open a particular website twice or maybe even three times.
OnlyOne comes to your rescue. Once you are on a particular website that you visit a lot like Gmail or Facebook, you could simply click on the extension icon to start OnlyOne. While surfing if you tend to open another instance of Facebook or Gmail, OnlyOne would automatically close the new version and redirect you to the existing one.
OnlyOne prevents you from opening many duplicate tabs and helps you to stay productive.
OnlyOne for Google Chrome [Download Link]