I found this out the hard way when, while messing with a plugin's code, I accidentally set all of my title tags to the same title. Luckily, I was able to catch it before google re-cached many of my posts and pages. I actually did this on two sites that were on the first page before at the same time. They both had drops in google by over a page and my sales off them have plummeted. Just a reminder to make all of your page's titles unique!