openrtb.schain.duplicate_node
What it means
Two adjacent nodes in the SupplyChain object carry the same asi (advertising system identifier) and sid (seller id), which looks like the same hop appended twice rather than two real hops.
Why it matters
Buyers audit schain against sellers.json hop by hop. A duplicated node makes the chain longer than the real supply path, fails path-length checks, and reads as either sloppy plumbing or deliberate padding. Unverifiable chains lose spend to paths that verify cleanly.
How to fix it
Append exactly one node per system that handled the request. The usual bug is a forwarder that appends its node without checking whether the upstream already added it.
Check your payload
Paste a bid request or response into the tester to see whether this code fires against it, or gate on the id in CI with the CLI: the id is stable, wording is not.
Back to the full diagnostic code reference.