Marketers who buy cost-per-performance ad inventory from networks could be paying for the same customer more than once, according to "The Hidden Cost of Pay-for-Performance Media" study by aQuantive's Atlas, MediaPostwrites. Different ad networks often take the credit for the same customer who, for example, visited those networks and saw ads on them for the same product before buying it online.

The study found an average duplication rate of 170 percent; that rate ranged from 26 percent to 379 percent. The potential average overpayment per campaign was $255,000, with a range between $15,000 and nearly $1 million.

Atlas served all the ads in the study, so it caught the duplications and the marketers didn't overpay; the report concludes that marketers should use third-party networks, such as Atlas, to serve cost-per-performance ads.