This error occurs when attempting to sign up a user with an email address already registered in Supabase Auth. Behavior varies depending on email confirmation settings.
The "user_already_exists" error is thrown by Supabase Auth when the email provided during registration is already associated with an existing user account. Supabase checks the uniqueness of the email field in the auth.users table, and if a duplicate email is detected, the registration process is halted. The error's behavior depends on your project's email confirmation settings. When email confirmation is disabled, Supabase returns a clear "User already registered" error. However, when email confirmation is enabled, Supabase returns an obfuscated/fake user object instead of a direct error message. This is a deliberate design decision to prevent account enumeration attacks, where attackers could determine which email addresses are registered. This security-focused approach means you need to handle existing users differently depending on your authentication configuration.
Navigate to your Supabase Dashboard → Authentication → Settings and check the "Confirm email" setting.
If disabled: Supabase will return a clear error message that you can catch and handle.
If enabled: You'll receive an obfuscated response and need to implement one of the workarounds below.
When email confirmation is enabled, check the user.identities array in the signup response:
const { data, error } = await supabase.auth.signUp({
email: '[email protected]',
password: 'password123'
});
if (data?.user) {
// Empty identities array means user already exists and confirmed email
if (data.user.identities && data.user.identities.length === 0) {
console.log('User already exists - redirect to login');
// Redirect to login page or show appropriate message
return;
}
// User created successfully
console.log('New user created');
}This is the most common client-side workaround for detecting duplicate signups.
If you need definitive error messages, use the Admin Auth API with your service role key on the server side:
// Server-side only - never expose service_role key to client
import { createClient } from '@supabase/supabase-js';
const supabaseAdmin = createClient(
process.env.NEXT_PUBLIC_SUPABASE_URL!,
process.env.SUPABASE_SERVICE_ROLE_KEY!, // Service role key
{
auth: {
autoRefreshToken: false,
persistSession: false
}
}
);
const { data, error } = await supabaseAdmin.auth.admin.createUser({
email: '[email protected]',
password: 'password123',
email_confirm: true
});
if (error) {
// Will receive clear error: "A user with this email address has already been registered"
console.error('User creation failed:', error.message);
}Warning: Only use this approach on the server side. Never expose your service role key to client-side code.
Create a Postgres function to check if an email exists before attempting signup:
-- Create a security definer function in your Supabase SQL Editor
CREATE OR REPLACE FUNCTION public.check_email_exists(email_to_check TEXT)
RETURNS BOOLEAN
LANGUAGE plpgsql
SECURITY DEFINER
AS $$
BEGIN
RETURN EXISTS (
SELECT 1 FROM auth.users WHERE email = email_to_check
);
END;
$$;Then call it from your client:
const { data, error } = await supabase.rpc('check_email_exists', {
email_to_check: '[email protected]'
});
if (data) {
// Email already exists - show login option
console.log('Email already registered. Please log in.');
} else {
// Proceed with signup
await supabase.auth.signUp({
email: '[email protected]',
password: 'password123'
});
}Update your signup UI to gracefully handle existing users:
async function handleSignup(email: string, password: string) {
const { data, error } = await supabase.auth.signUp({
email,
password
});
if (error) {
// Email confirmation disabled - direct error
if (error.message.includes('User already registered')) {
showMessage('This email is already registered. Please log in or reset your password.');
redirectToLogin();
return;
}
showError(error.message);
return;
}
// Email confirmation enabled - check identities
if (data?.user?.identities?.length === 0) {
showMessage('An account with this email already exists. Please log in.');
redirectToLogin();
return;
}
// Success
showMessage('Account created! Please check your email to confirm.');
}Provide clear options to either log in or reset password when detecting existing users.
Why Obfuscated Responses?
Supabase deliberately returns fake user objects when email confirmation is enabled to prevent account enumeration attacks. This security measure prevents malicious actors from determining which email addresses are registered by attempting signups.
Disabling Email Confirmation Trade-offs
While disabling email confirmation gives you clear error messages, it removes an important security layer. Email confirmation verifies that users own the email addresses they register with. Only disable this if you have alternative verification mechanisms.
Rate Limiting Considerations
Implement rate limiting on your signup endpoints to prevent abuse, especially if you're using custom email existence checks. Multiple rapid checks could be used for email enumeration.
Alternative: OAuth Providers
If your use case allows, consider using OAuth providers (Google, GitHub, etc.) which handle duplicate account scenarios automatically. Supabase will link accounts or show appropriate errors based on the provider's response.
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