A little known and therefore not so widely enforced AdWords rule is Google's policy on double serving. You can't double serve on AdWords. This actually means you shouldn't double serve on AdWords although many people do.
What's double serving? Double serving is having two different AdWords creatives listing for the same keyword and for the same site. In fact, double serving has a broader ruling than that insofar as sites owned by the same company which sells the same thing are not allowed to run side-by-side in AdWords. Their AdWords accounts should be linked and only one AdWord should run. This rule is there to stop people simply setting up different sites and effectively owning the right hand-side of Google.
The catch is that it's not very clear that similar sites may be linked. Not every webmaster knows which big brand owns which other - heck, many webmasters would struggle to separate companies from brands.
The key is to stay in the know. If you have a client who, for example, is into loans - a very competitive PPC market - the know the other big loan players. Know which banks have a branded loan company in their umbrella. If you see branded loan offering running alongside the main bank for a loan keyword then that's a breach of Google's double serving policies.
The thing to do is to kick up a fuss. It won't make you popular with any of your campaign managers at Google but its a great way to cut swathes through your clients competition.
Sometimes the arguement comes up that the two sites look different. That's not enough. The sites must have different products and services.