Auction duplication

Auction duplication is the situation in which one ad impression reaches a buyer several times, through different SSPs, resellers or wrappers, so the buyer sees what looks like many opportunities but can win only one.

Header bidding lets a publisher call many SSPs at once, and each SSP may pass the request to other sellers. The IAB Tech Lab calls this a multi-hop supply chain and notes that more than two or three hops is rare and usually suboptimal (IAB Tech Lab, Programmatic Auction Definitions). Every copy costs the DSP processing time and queries per second. A buyer can also end up bidding against itself, which pushes prices up.

OpenRTB gives buyers a tool to spot copies. The source.tid field is a transaction ID that should be the same for every participant, even across several exchanges (IAB Tech Lab, OpenRTB 2.6, section 3.2.2). The schain object shows the path each copy took. Privacy pulls the other way. Prebid.js has disabled shared transaction IDs by default since version 8, because they let DSPs correlate requests from different sources; publishers can turn them back on with enableTIDs (Prebid.org, setConfig).

Example: A DSP receives five requests within 30 ms for one slot on a sports site: two directly from SSPs and three through resellers. All five carry the same source.tid. The DSP bids only on the copy with the shortest schain, so it answers one request instead of five. The trade-offs are covered in header bidding vs server-side bidding.

Related terms: supply path optimization, SupplyChain object, header bidding, QPS, sellers.json, demand path optimization.

Authoritative source: IAB Tech Lab, OpenRTB 2.6, section 3.2.2 "Object: Source" (accessed 9 November 2026).