When working with AWS Cognito for authentication, it’s not uncommon to encounter specific errors that can disrupt the flow. One such error is: "Already found an entry for username Facebook_10155611263153532". This article explains the reasons for this error, explores possible solutions, and discusses best practices to avoid similar issues.

Understanding the Error

The error message "Already found an entry for username Facebook_10155611263153532" typically occurs when there is a clash of identifiers in your Cognito user pool. Specifically, this involves scenarios where federated identities such as Facebook or Google logins are being used.

The string `Facebook_10155611263153532` represents a unique identifier generated by Cognito for a user who has logged in using their Facebook account. Here’s a closer look into the structure of this identifier:

• `Facebook` indicates the identity provider (IdP).
• `10155611263153532` is the unique Facebook identifier for the user.

Possible Reasons for the Error

1. Duplicate External Identifiers: If you have multiple Cognito User Pools and the same external identity (like a particular Facebook account) is linked to accounts in different user pools, you might face such collisions.
2. Incorrect Custom Attribute Mapping: If custom attributes are not mapped correctly or consistently across platforms (like different environments or apps), duplicate acclamations might occur.
3. Inconsistent IdP Configuration: Misconfiguration in identity providers, such as incorrect settings in the Facebook developer console or an improper integration with AWS Cognito, might cause redundant entries.

Troubleshooting and Solutions

1. Verify User Pool Configurations

First, identify if the problematic entry exists within the user pool:

• Application ID: Match with what is configured in AWS Cognito.
• Secret Keys: Ensure they are accurately implemented in your backend.
• OAuth Parameters: Verify that OAuth scopes and endpoints are correctly defined.
• Ensure consistent naming conventions across development, staging, and production.
• Check mappings in the Cognito console under "Attribute mapping."
• Logging and Monitoring: Utilize AWS CloudWatch to keep an eye on authentication and authorization logs to spot patterns leading to such conflicts.
• User Data Backup: Before modifying user entries, back up important data to avoid accidentally losing significant user information.
• Prevention Mechanisms: Implement mechanisms to detect and prevent duplicate users at the application logic level even before they reach Cognito.