Every time you open a new tab or navigate to a new page, this extension automatically checks for duplicates in the same window.
It'll switch to the original tab and close the new one then (A notification is displayed in Chrome. Clicking this notification will reopen it)
Click version history to find the newest build by me.
Have a look at Unique Tabs in Chrome Web Store for the original extension by Joey Aghion. Click +Add to Chrome to install it.
Feedback and forks welcome.