AWS Cognito: Best practice to handle same user (with same email address) signing in from different identity providers (Google, Facebook)

When signing in a user with the same email address through the Google and Facebook identity providers, AWS Cognito creates multiple entries in the user pool, one entry per identity provider used:

I have used the example code provided in this tutorial to set up AWS Cognito: The Complete Guide to User Authentication with the Amplify Framework

• How can I create just one user instead of multiple users?
• Is it possible to have AWS Cognito automatically combine (federate) the entries from multiple providers into one entry or should AWS Lambda functions be used to accomplish this?

Yes. You can do it by using AdminLinkProviderForUser https://docs.aws.amazon.com/cognito-user-identity-pools/latest/APIReference/API_AdminLinkProviderForUser.html

The idea is:

1. In PreSignUp lambda hook, we Link Provider to User if User already signed up. E.g:

import CognitoIdentityServiceProvider from 'aws-sdk/clients/cognitoidentityserviceprovider'

const cognitoIdp = new CognitoIdentityServiceProvider()
const getUserByEmail = async (userPoolId, email) => {
 const params = {
   UserPoolId: userPoolId,
   Filter: `email = "${email}"`
 }
 return cognitoIdp.listUsers(params).promise()
}

const linkProviderToUser = async (username, userPoolId, providerName, providerUserId) => {
 const params = {
   DestinationUser: {
     ProviderAttributeValue: username,
     ProviderName: 'Cognito'
   },
   SourceUser: {
     ProviderAttributeName: 'Cognito_Subject',
     ProviderAttributeValue: providerUserId,
     ProviderName: providerName
   },
   UserPoolId: userPoolId
 }

 const result = await (new Promise((resolve, reject) => {
   cognitoIdp.adminLinkProviderForUser(params, (err, data) => {
     if (err) {
       reject(err)
       return
     }
     resolve(data)
   })
 }))

 return result
}

exports.handler = async (event, context, callback) => {
 if (event.triggerSource === 'PreSignUp_ExternalProvider') {
   const userRs = await getUserByEmail(event.userPoolId, event.request.userAttributes.email)
   if (userRs && userRs.Users.length > 0) {
     const [ providerName, providerUserId ] = event.userName.split('_') // event userName example: "Facebook_12324325436"
     await linkProviderToUser(userRs.Users[0].Username, event.userPoolId, providerName, providerUserId)
   } else {
     console.log('user not found, skip.')
   }

 }
 return callback(null, event)
}

2. Then when user use OAuth with Facebook/Google with User Pool, the Pool will return this User linked.

Note: You may see 2 records in User Pool UI, but when access User record detail, They already merged.