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Google will soon block the creation of duplicate Lookalike user lists in the Google Ads API, returning an error code for any violations.
A quiet but important change is coming to the Google Ads API that will affect how advertisers and developers create Lookalike user lists, especially for Demand Gen campaigns.
What’s changing. Google will enforce a uniqueness check on Lookalike user lists, blocking duplicate lists with the same seed lists, expansion level, and country targeting. Attempts to create a duplicate will return an API error after April 30.
Why we care. If you use automated scripts or third-party tools to generate audience lists, an unhandled error could quietly break your campaign workflows if you don’t update integrations 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_LOOKALIKEerror code in v24 and above, orRESOURCE_ALREADY_EXISTSin earlier versions
Bottom line. This is a housekeeping change to keep Google’s systems stable, but the April 30 deadline is firm. If you manage campaigns programmatically, treat this as a technical to-do before the end of April.
Google’s announcement. Upcoming changes to Lookalike user lists in the Google Ads API, starting April 30, 2026
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