In many cases, advertisers use multiple traffic sources like affiliate platforms, paid search and paid social to generate more sales. Many of these traffic sources use their own tracking methods which can lead to a sale being claimed by different traffic sources. For example:
An advertiser is running a Facebook campaign and a campaign on a affiliate platform. A visitor clicks on a FB ad and visits an affiliate website and ends up with a FB cookie and a publisher network cookie on their computer. As a result, both parties claim to have generated the last click to the sale. In such cases, deduplication is necessary to prevent an advertiser from paying twice for the same sale. This situation is undesirable for both advertisers and publishers; advertisers pay too much and publishers notice that their sale has been rejected without understanding whether this is justified or not.