Double-Serving is sneaky and bad! I don't like it and neither does Google. Obvious double-serving occurs when you have more than one Google Adwords account sending ads to the same domain or subdomain. Some sneaky people do this to gain an advantage of having more than one ad in the auction and more than one ad to appear in the top 3.
Sneakier people create multiple domains for basically the same store. So, the ads are going to separate domains, technically, but it's all the same products. OR, you have affiliates with a "squeeze" page that then leads back to the first domain. All sneaky. All not liked.
These tactics can get your accounts suspended or even banned. Google isn't trying to ruin your business. They are just trying to get you to play by the rules. If a user is completing a search for "ski gear" and all of the ads inevitably point to the exact same stuff — That user is not getting a good experience or choice or options. Good experience = Long time user = More revenue over time!!
So, play by the rules peeps. It's not that hard. If you have a good site and a good product(s), you can be successful without cheating.