I was trying to create Unity quicklists for Gmail. But if the Gmail was already opened, it will still create another tab and load the web page, rather than switching to the existing.
I even tried installing the 'Gmail App' from the app store and nothing happened.
I looked at the various command line switches, I could not find anything.
http://peter.sh/experiments/chromium-command-line-switches/
The other clumsy solution was to install an addon called Prevent Duplicate Tabs
https://chrome.google.com/webstore/detail/ljnhndkfkbcfmfdkfaahkfmiiibaldhd
It removed the duplicate tabs but it still had to load the page than switching to it. It also doesn't work with forwarding URLs like Google Docs
https://docs.google.com/?pli=1#home.
On a side note wasn't this meant to be a feature for web apps, a buzz word today.
Best Answer
The Chrome extension, Clutter Free, does exactly what you asked:
Disclaimer: I'm the author for this extension. I too was looking for this solution, and ended up making the extension.