What is bid duplication in programmatic?
Bid duplication happens when the same ad impression is offered to a buyer multiple times through different SSPs and resellers, so the buyer's DSP sees what looks like several opportunities but is really one. It inflates bid-request volumes, raises costs through bidding against yourself, and adds fees. Supply path optimization aims to reduce it.
Header bidding lets publishers call many SSPs, and each may resell through others, so a single impression can generate dozens of bid requests. Bid duplication makes it hard for buyers to know true reach and can distort win rate and auction dynamics.
Buyers counter with supply path optimization, transaction-ID deduplication (a shared ID for each impression across sellers), and preferring direct seller paths listed in ads.txt.