Google is blocking duplicate Lookalike audience lists

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

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