A quiet but important change is coming to the Google Ads API that will affect how advertisers and developers create Lookalike user lists — particularly those running Demand Gen campaigns.
What’s changing. Google will begin enforcing a uniqueness check on Lookalike user lists, preventing the creation of duplicate lists that share the same seed lists, expansion level, and country targeting. Attempts to create a duplicate after April 30 will return an API error.
Why we care. Teams using automated scripts or third-party tools to programmatically generate audience lists, an unhandled error could quietly break campaign workflows if integrations aren’t updated in time.
What you need to do.
Audit existing Lookalike lists and reuse ones that already match your intended configuration rather than creating new ones
Update your API error handling to catch the new DUPLICATE_LOOKALIKE error code in v24 and above, or RESOURCE_ALREADY_EXISTS in earlier versions
The bottom line. This is a housekeeping change designed to keep Google’s systems stable — but the April 30 deadline is firm. Developers and agencies managing campaigns programmatically should treat this as a technical to-do before the end of April.
Dig deeper. Upcoming changes to Lookalike user lists in the Google Ads API, starting April 30, 2026