Thursday, March 19, 2026


Google Ads is committed to providing a reliable and efficient environment for all advertisers. To ensure that our systems remain stable and that resources are allocated fairly, we are introducing a uniqueness check for [Lookalike user lists](https://developers.google.com/google-ads/api/docs/remarketing/audience-segments/lookalike-audiences.md.txt?utm_source=googleblog-adsdevs&utm_medium=blog&utm_campaign=adr-duplicatelookalikeuserlistchangesmar2026&utm_content=readmore-lookalike) that prevents the creation of duplicate lists. These changes primarily impact [Demand Gen campaigns](https://developers.google.com/google-ads/api/docs/demand-gen/audience-targeting.md.txt?utm_source=googleblog-adsdevs&utm_medium=blog&utm_campaign=adr-duplicatelookalikeuserlistchangesmar2026&utm_content=readmore-demandgen) and will begin to be enforced starting on **April 30, 2026**.


A [Lookalike user list](https://developers.google.com/google-ads/api/reference/rpc/v23/LookalikeUserListInfo.md.txt?utm_source=googleblog-adsdevs&utm_medium=blog&utm_campaign=adr-duplicatelookalikeuserlistchangesmar2026&utm_content=readmore-lookalikeuserlist) is considered a duplicate if it shares all of the following attributes with an existing list:

- **Seed lists:** The underlying source audiences used to build the Lookalike.
- **Expansion level:** The degree of similarity to the seed lists.
- **Country targeting:** The geographical reach of the Lookalike list.

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If an advertiser attempts to create a duplicate list after April 30, 2026, the [Google Ads API](https://developers.google.com/google-ads/api?utm_source=googleblog-adsdevs&utm_medium=blog&utm_campaign=adr-duplicatelookalikeuserlistchangesmar2026&utm_content=readmore-gaa) will return the error code `DUPLICATE_LOOKALIKE` for v24 and beyond. For earlier versions, the error will be `RESOURCE_ALREADY_EXISTS`.

### **What this means for you**


For the majority of advertisers, these changes will go unnoticed, as most current usage falls well within these new boundaries. However, to ensure a smooth transition, we recommend the following:

- **Reuse audiences:** Instead of creating a new, identical Lookalike list, verify if an existing one with the same configuration can be reused in your campaigns.
- **Update error handling:** If you use the Google Ads API, ensure your integrations are prepared to handle the new error starting on April 30, 2026: v24 will return the `DUPLICATE_LOOKALIKE` error code and all previous versions will return `RESOURCE_ALREADY_EXISTS`.


If you have any questions or want to discuss this post, please reach out to us on our ["Google Advertising and Measurement Community" Discord server](http://goo.gle/ads-and-measurement-discord).

![Portrait of the author](https://developers.google.com/static/ads/blog/images/AVvXsEhMd1fdKsHQ-Ld1S2S7p1QnHKGIsAxgkzDAnAZ4Ennu3TL968-LZlrDhnCAtVhhh-UUMIKK6ecAadUoYY2_S-nU6HNVWDZK4vE28WYzIilQUhe8Fh_FMOaFo5sahlfpyIV7j_sR8xS-HtM_KU_deoWN91Y3dk21HE65O2lzO4fd8aOAFTxqpd2-rVhO5XM.jpg) - Dora Sun - Google Ads API Team

**Labels:**

[audiences](https://developers.google.com/ads/blog/feeds/labels/audiences.md.txt),

[demand-gen](https://developers.google.com/ads/blog/feeds/labels/demand-gen.md.txt),

[google-ads-api](https://developers.google.com/ads/blog/feeds/labels/google-ads-api.md.txt)