Integrating Social SSO into your React app is supposed to make life easier. But if you aren't careful with how your backend handles identities, you will create a massive headache for yourself.

Today, I realized my AWS Cognito integration was silently creating duplicate accounts.

The Problem: I had users who originally signed up with an email and password. When I added "Sign in with Google", Cognito treated those returning users as completely new entities because they had a new internal sub ID. My DynamoDB tables were fragmenting. Worse, my AI Engine (Amazon Bedrock) was being invoked from scratch for these returning users, driving up my cloud bill unnecessarily and destroying loading times.

The Architectural Fix (Lambda Interceptor)

I couldn't let Cognito dictate my database schema.…