Cognito: Unexpected behavior when creating a group that already exists

[NateDimick]

I recently jumped from version 4.7 to 2026.5 (2026.5.3) and found a regression in cognito CreateGroup behavior. Creating a group that exists does not cause a GroupExistsException as expected, and instead it recreates the group and removes the group from all users who had been assigned that group previously.

observed behavior example:

ndimick@ndimick-lt2:~$ aws cognito-idp admin-list-groups-for-user --user-pool-id us-east-1_49bc13b0a98549f0ad4b65e54536b9bf --username my-user --profile localstack
{
    "Groups": [
        {
            "GroupName": "MY_GROUP_NAME",
            "UserPoolId": "us-east-1_49bc13b0a98549f0ad4b65e54536b9bf",
            "LastModifiedDate": "2026-06-10T11:45:32-04:00",
            "CreationDate": "2026-06-10T11:45:32-04:00"
        }
    ]
}
ndimick@ndimick-lt2:~$ aws cognito-idp create-group --profile localstack --user-pool-id us-east-1_49bc13b0a98549f0ad4b65e54536b9bf --group-name MY_GROUP_NAME
{
    "Group": {
        "GroupName": "MY_GROUP_NAME",
        "UserPoolId": "us-east-1_49bc13b0a98549f0ad4b65e54536b9bf",
        "LastModifiedDate": "2026-06-10T11:47:43-04:00",
        "CreationDate": "2026-06-10T11:47:43-04:00"
    }
}
ndimick@ndimick-lt2:~$ aws cognito-idp admin-list-groups-for-user --user-pool-id us-east-1_49bc13b0a98549f0ad4b65e54536b9bf --username my-user --profile localstack
{
    "Groups": []
}

expected behavior:

ndimick@ndimick-lt2:~$ aws cognito-idp create-group --user-pool-id us-east-1_XXXXXX --group-name MY_GROUP_NAME

An error occurred (GroupExistsException) when calling the CreateGroup operation: A group with the name already exists.

[HarshCasper]

Hi — The fix is merged and live in the localstack/localstack-pro:dev image!

To try it: docker pull localstack/localstack-pro:dev (it'll roll into the next stable release too).

Give it a spin when you get a chance and let us know if anything still looks off. Happy to dig in further. Thanks for the clear report and the reproduction steps, they made this much easier to fix! 🙏

[NateDimick]

Awesome! I'll have to wait for the next stable release, but thanks for handling this so quickly!