Google Ads will enforce new rules to prevent the creation of duplicate Lookalike user lists via the API starting April 30, 2026. The goal is to ensure system reliability and fair resource allocation for advertisers, especially for Demand Gen campaigns.
A Lookalike list will now be flagged as a duplicate if it matches an existing list on seed audiences, expansion level, and country targeting.
From April 30, advertisers trying to create a duplicate will receive the new DUPLICATE_LOOKALIKE error in API v24 and the existing RESOURCE_ALREADY_EXISTS code in previous versions.
Most advertisers are unlikely to be affected, but Google recommends reusing existing audiences and updating API integrations to handle the new errors.
Read the official announcement here: Upcoming changes to Lookalike user lists in the Google Ads API.