Google Ads is implementing a uniqueness check for Lookalike user lists to prevent duplicate list creation, primarily affecting Demand Gen campaigns starting April 30, 2026. A Lookalike user list is considered duplicate if it matches an existing list in three key attributes: seed lists (source audiences), expansion level (similarity degree), and country targeting (geographical reach).
Impact and Enforcement
After April 30, 2026, attempts to create duplicate Lookalike lists will trigger errors in the Google Ads API:
- v24 and later: Returns error code
DUPLICATE_LOOKALIKE - Earlier versions: Returns error code
RESOURCE_ALREADY_EXISTS
Recommendations for Advertisers
Most advertisers will not be affected as current usage generally complies with these rules. To ensure a smooth transition:
- Reuse audiences: Check if an existing Lookalike list with the same configuration can be reused instead of creating a new one.
- Update error handling: Prepare API integrations to handle the new error codes appropriately from April 30, 2026.
For further discussion or questions, Google Ads suggests engaging via their “Google Advertising and Measurement Community” Discord server.
