By default, the WaaS API only allows one account per email. If a user attempts to login using a different method but the same email as before, they will receive an EmailAlreadyInUse error. For example: if the user created their account using Google Sign In and then attempts to sign in with Email + OTP using the same method, they will receive this error. By default (if using the default SequenceLoginWindow), the SDK will automatically open the FederatedAuthPopupPanel prefab located at SequenceFrontend > Prefabs > FederatedAuthPopupPanel. This prefab can be customized to your linking or replaced in the SequenceLoginWindow prefab. This panel will explain to the user that duplicate login methods are not allowed and prompt them to return to the login screen and sign in with their associated login method (or a different email altogether).
If EnableMultipleAccountsPerEmail is enabled in SequenceConfig the FederatedAuthPopupPanel will give the user the option to create another account associated with their email.
Once the user has logged into their account; the SDK will automatically make a FederateAccount request (see SequenceLogin). This will associate the failed login method with that email as well so that the user may sign in with either in the future. Using our example above, the user would now be able to sign into their account using Google Sign In or Email + OTP.

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Manually Federating Accounts

You may want to add a button to your app allowing the user to associate an additional login method with their email (especially if you are using Guest Login). You’ll want get a reference to SequenceLogin and then call the appropriate FederateAccount method for the login method. 
SequenceLogin login = SequenceLogin.GetInstanceToFederateAuth(walletAddress);

// PlayFab
login.FederateAccountPlayFab(titleId, sessionTicket, email, walletAddress);

// OIDC (Social)
login.FederateAccountSocial(idToken, loginMethod, walletAddress);

// Guest
login.FederateAccountGuest(walletAddress);

// Email
login.Login(email);
// Later ... Once you've received the OTP code from the user
login.FederateAccountEmail(email, code, walletAddress);
where walletAddress is the address of the SequenceWallet you retrieved after authenticating the user.

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Re-using the Login Window Boilerplate

To allow a user to federate/link their accounts manually using the default LoginPanel, simply call SetConnectedWalletAddress on your SequenceLogin instance or call SequenceLogin.GetInstanceToFederateAuth with the currently authenticated wallet address provided. 
SequenceLogin login = SequenceLogin.GetInstance();
login.SetConnectedWalletAddress(authenticatedSequenceWalletAddress);

// or

SequenceLogin.GetInstanceToFederateAuth(authenticatedSequenceWalletAddress);
This will configure your SequenceLogin instance to federate the accounts instead of creating a new session.
Once the user has logged out, don’t forget to call RemoveConnectedWalletAddress() to re-configure SequenceLogin to create new sessions again.
e.g. 
private void OnDropSessionCompleteHandler(string sessionId) {
    if (sessionId == Wallet.SessionId)
    {
        SequenceLogin.GetInstance().RemoveConnectedWalletAddress();
        SceneManager.LoadScene("LoginScene"); // Re-open your scene or UI to allow the user to log back in and create a new session here
    }
}

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Removing Federated Accounts

If you ever want to remove a federated account association, this can be done with the IWallet.RemoveFederatedAccount method. 
_wallet.OnFederatedAccountRemovedComplete += (accountIdString, success) => {
    if (success) 
    {
        Debug.Log($"Successfully removed {accountIdString} as a login method");
        // Do something
    }else 
    {
        // Handle failure
    }
};

bool success = await _wallet.RemoveFederatedAccount(account);