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# Post Authentication - User Management -  Account deduplication
Source: https://supertokens.com/docs/post-authentication/user-management/account-deduplication

## Overview

Users may forget the initial method they used to sign up and may create multiple accounts with the same email ID - leading to a poor user experience.
Preventing this from happening refers to account deduplication.

As an example, assume that your app has Google and GitHub login.
There exists a user who had signed up with Google using their email ID - `user@gmail.com`.
If this user then tries to sign up with GitHub, which has this same email (`user@gmail.com`), your app disallows this. It shows them an appropriate message like "Your account already exists via Google sign in. Please use that instead."

### Comparison to account linking

Related to this problem is also the concept of account linking.
The difference is that whilst deduplication prevents duplicate sign ups, account linking allows duplicate sign ups, but implicitly merges the duplicate accounts into one.

## Steps

### 1. Override the authentication recipes

The approach to implementing account deduplication is to override the backend functions / APIs.
This way, you can check if a user already exists and return an error to the frontend if the condition is true.

<Tabs>
<Tab value="nodejs">

```tsx

let recipeList = [
    Passwordless.init({
        contactMethod: "EMAIL", // REMOVE_FROM_OUTPUT
        flowType: "USER_INPUT_CODE_AND_MAGIC_LINK", // REMOVE_FROM_OUTPUT
        override: {
            apis: (originalImplementation) => {
                return {
                    ...originalImplementation,
                    createCodePOST: async function (input) {
                        if ("email" in input) {
                            let existingUsers = await supertokens.listUsersByAccountInfo(input.tenantId, {
                                email: input.email
                            });
                            if (existingUsers.length === 0) {
                                // this means this email is new so we allow sign up
                                return originalImplementation.createCodePOST!(input);
                            }
                            if (existingUsers.find(u =>
                                u.loginMethods.find(lM => lM.hasSameEmailAs(input.email) && lM.recipeId === "passwordless") !== undefined)) {
                                // this means that the existing user is a passwordless login user. So we allow it
                                return originalImplementation.createCodePOST!(input);
                            }
                            return {
                                status: "GENERAL_ERROR",
                                message: "Seems like you already have an account with another method. Please use that instead."
                            }
                        }
                        // phone number based login, so we allow it.
                        return originalImplementation.createCodePOST!(input);
                    },
                }
            }
        }
    }),
    ThirdParty.init({
        override: {
            functions: (originalImplementation) => {
                return {
                    ...originalImplementation,
                    signInUp: async function (input) {
                        let existingUsers = await supertokens.listUsersByAccountInfo(input.tenantId, {
                            email: input.email
                        });
                        if (existingUsers.length === 0) {
                            // this means this email is new so we allow sign up
                            return originalImplementation.signInUp(input);
                        }
                        if (existingUsers.find(u =>
                            u.loginMethods.find(lM => lM.hasSameThirdPartyInfoAs({
                                id: input.thirdPartyId,
                                userId: input.thirdPartyUserId
                            }) && lM.recipeId === "thirdparty") !== undefined)) {
                            // this means we are trying to sign in with the same social login. So we allow it
                            return originalImplementation.signInUp(input);
                        }
                        // this means that the email already exists with another social or passwordless login method, so we throw an error.
                        throw new Error("Cannot sign up as email already exists");
                    }
                }
            },
            apis: (originalImplementation) => {
                return {
                    ...originalImplementation,
                    signInUpPOST: async function (input) {
                        try {
                            return await originalImplementation.signInUpPOST!(input);
                        } catch (err: any) {
                            if (err.message === "Cannot sign up as email already exists") {
                                // this error was thrown from our function override above.
                                // so we send a useful message to the user
                                return {
                                    status: "GENERAL_ERROR",
                                    message: "Seems like you already have an account with another method. Please use that instead."
                                }
                            }
                            throw err;
                        }
                    }
                }
            }
        }
    })
]
```

</Tab>
<Tab value="go">

```go
							// so we send a useful message to the user
							return tpmodels.SignInUpPOSTResponse{
								GeneralError: &supertokens.GeneralErrorResponse{
									Message: "Seems like you already have an account with another method. Please use that instead.",
								},
							}, nil
						}

						return resp, err
					}

					return originalImplementation
				},
			},
		}),
	}
}
```

</Tab>
<Tab value="python">

```python
from typing import Any, Dict, Optional, Union

from supertokens_python import InputAppInfo, init
from supertokens_python.asyncio import list_users_by_account_info
from supertokens_python.recipe import passwordless, thirdparty
from supertokens_python.recipe.passwordless.interfaces import (
    APIInterface as PasswordlessAPIInterface,
)
from supertokens_python.recipe.passwordless.interfaces import (
    APIOptions as PasswordlessAPIOptions,
)
from supertokens_python.recipe.session.interfaces import SessionContainer
from supertokens_python.recipe.thirdparty.interfaces import (
    APIInterface as ThirdPartyAPIInterface,
)
from supertokens_python.recipe.thirdparty.interfaces import (
    APIOptions as ThirdPartyAPIOptions,
)
from supertokens_python.recipe.thirdparty.interfaces import (
    RecipeInterface,
)
from supertokens_python.recipe.thirdparty.provider import Provider, RedirectUriInfo
from supertokens_python.recipe.thirdparty.types import (
    RawUserInfoFromProvider,
    ThirdPartyInfo,
)
from supertokens_python.types import GeneralErrorResponse
from supertokens_python.types.base import AccountInfoInput

def override_thirdparty_functions(original_implementation: RecipeInterface):
    original_sign_in_up = original_implementation.sign_in_up

    async def sign_in_up(
        third_party_id: str,
        third_party_user_id: str,
        email: str,
        is_verified: bool,
        oauth_tokens: Dict[str, Any],
        raw_user_info_from_provider: RawUserInfoFromProvider,
        session: Optional[SessionContainer],
        should_try_linking_with_session_user: Union[bool, None],
        tenant_id: str,
        user_context: Dict[str, Any],
    ):
        existing_users = await list_users_by_account_info(
            tenant_id, AccountInfoInput(email=email)
        )
        if len(existing_users) == 0:
            # this means this email is new so we allow sign up
            return await original_sign_in_up(
                third_party_id,
                third_party_user_id,
                email,
                is_verified,
                oauth_tokens,
                raw_user_info_from_provider,
                session,
                should_try_linking_with_session_user,
                tenant_id,
                user_context,
            )

        if any(
            any(
                lm.recipe_id == "thirdparty"
                and lm.has_same_third_party_info_as(
                    ThirdPartyInfo(third_party_user_id, third_party_id)
                )
                for lm in user.login_methods
            )
            for user in existing_users
        ):
            # this means we are trying to sign in with the same social login. So we allow it
            return await original_sign_in_up(
                third_party_id,
                third_party_user_id,
                email,
                is_verified,
                oauth_tokens,
                raw_user_info_from_provider,
                session,
                should_try_linking_with_session_user,
                tenant_id,
                user_context,
            )

        # this means that the email already exists with another social login method.
        # so we throw an error.
        raise Exception("Cannot sign up as email already exists")

    original_implementation.sign_in_up = sign_in_up

    return original_implementation

def override_thirdparty_apis(original_implementation: ThirdPartyAPIInterface):
    original_sign_in_up_post = original_implementation.sign_in_up_post

    async def sign_in_up_post(
        provider: Provider,
        redirect_uri_info: Optional[RedirectUriInfo],
        oauth_tokens: Optional[Dict[str, Any]],
        session: Optional[SessionContainer],
        should_try_linking_with_session_user: Union[bool, None],
        tenant_id: str,
        api_options: ThirdPartyAPIOptions,
        user_context: Dict[str, Any],
    ):
        try:
            return await original_sign_in_up_post(
                provider,
                redirect_uri_info,
                oauth_tokens,
                session,
                should_try_linking_with_session_user,
                tenant_id,
                api_options,
                user_context,
            )
        except Exception as e:
            if str(e) == "Cannot sign up as email already exists":
                return GeneralErrorResponse(
                    "Seems like you already have an account with another social login provider. Please use that instead."
                )
            raise e

    original_implementation.sign_in_up_post = sign_in_up_post
    return original_implementation

def override_passwordless_apis(original_implementation: PasswordlessAPIInterface):
    original_create_code_post = original_implementation.create_code_post

    async def create_code_post(
        email: Union[str, None],
        phone_number: Union[str, None],
        session: Optional[SessionContainer],
        should_try_linking_with_session_user: Union[bool, None],
        tenant_id: str,
        api_options: PasswordlessAPIOptions,
        user_context: Dict[str, Any],
    ):
        if email is not None:
            existing_users = await list_users_by_account_info(
                tenant_id, AccountInfoInput(email=email)
            )
            if len(existing_users) == 0:
                # this means this email is new so we allow sign up
                return await original_create_code_post(
                    email,
                    phone_number,
                    session,
                    should_try_linking_with_session_user,
                    tenant_id,
                    api_options,
                    user_context,
                )

            if any(
                user.login_methods
                and any(
                    lm.recipe_id == "passwordless" and lm.has_same_email_as(email)
                    for lm in user.login_methods
                )
                for user in existing_users
            ):
                # this means that the existing user is a passwordless login user. So we allow it
                return await original_create_code_post(
                    email,
                    phone_number,
                    session,
                    should_try_linking_with_session_user,
                    tenant_id,
                    api_options,
                    user_context,
                )

            return GeneralErrorResponse(
                "Seems like you already have an account with another method. Please use that instead."
            )

        # phone number based login, so we allow it.
        return await original_create_code_post(
            email,
            phone_number,
            session,
            should_try_linking_with_session_user,
            tenant_id,
            api_options,
            user_context,
        )

    original_implementation.create_code_post = create_code_post
    return original_implementation

init(
    app_info=InputAppInfo(api_domain="...", app_name="...", website_domain="..."),
    framework="...",  # type: ignore
    recipe_list=[
        passwordless.init(
            contact_config=...,  # type: ignore
            flow_type="...",  # type: ignore
            override=passwordless.InputOverrideConfig(
                apis=override_passwordless_apis,
            ),
        ),
        thirdparty.init(
            override=thirdparty.InputOverrideConfig(
                apis=override_thirdparty_apis, functions=override_thirdparty_functions
            )
        ),
    ],
)
```

</Tab>
</Tabs>

In the above code snippet, override the `signInUpPOST` (third party recipe) and the `createCodePOST` (passwordless recipe) API as well as the `signInUp` recipe function.

The frontend calls the `signInUpPOST` API after the user returns to the app from the third-party provider's login page. The API then exchanges the auth code with the provider and calls the `signInUp` function with the user's email and third-party info.

The system calls the `createCodePOST` API when the user enters their email, or phone number during passwordless login. This API generates the passwordless OTP / link and sends it to the user's email / phone.

The `signInUp` recipe function is overridden to:
- Get all ThirdParty or Passwordless users that have the same input email.
- If no users exist with that email, it means that this is a new email and the system calls the `originalImplementation` function to create a new user.
- If instead, a user exists, but has the same `thirdPartyId` and `thirdPartyUserId`, implying that this is a sign in (for example a user who had signed up with Google is signing in with Google), the operation proceeds by calling the `originalImplementation` function.
- If neither of the conditions above match, it means that the user is trying to sign up with a third party provider whilst they already have an account with another provider or via passwordless login. Here, the system throws an error with some custom message.

Finally, the `signInUpPOST` API is overridden to catch that custom error and return a [general error status](/docs/references/backend-sdks/api-overrides#error-management) to the frontend with a message displayed to the user in the sign in form.

The `createCodePOST` API is also overridden to perform similar checks:
- If the input is phone number based, then the system calls the `originalImplementation` function allowing sign up or sign in. This is OK since social login is always email based, there is no scope of duplication.
- Otherwise, get all ThirdParty or Passwordless users that have the same input email.
- If no users exist with that email, it means that this is a new email and the system calls the `originalImplementation` function to create a new user.
- Else, check if the existing user is not a Third Party login user, implying that it's a Passwordless login user. Here, the `originalImplementation` function is also called to allow the user to sign in.
- If neither of the conditions above match, it means that the user is trying to sign up with passwordless login whilst they already have an account with a third party provider. Here, the system returns an appropriate message to display on the frontend.

:::info Multi Tenancy

For a multi tenant setup, the customisations above ensure that multiple accounts with the same email don't exist within a single tenant. To ensure no duplication across all tenants, when fetching the list of existing users, loop through all tenants in the app. You can fetch them by using the `listAllTenants` function of the multi tenancy recipe.

:::