Google announced on Google+ that they have added a new feature named Order IDs that allow you to add a unique identifier into your AdWords conversion tags to help reduce duplicate conversion data.
In short, advertisers added website code to make sure conversion tags didn't fire again when people refreshed or returned to confirmation pages. The example Google gave was, if you’re a hotel brand, you may notice customers returning to their booking confirmation pages, perhaps to find the reservation number or room check-in time the day before a trip.
If you include an Order ID into conversion tags, AdWords will not count these subsequent conversions with the same Order ID.
Google said, "by minimizing duplicate conversions and reviewing more accurate conversion data, you can make better budget and bidding decisions for your campaigns."
Here is a help document with how to implement the new Order IDs.
Forum discussion at Google+.
