# Signup Duplicate Phone Number Check

## Problem
When a user tries to sign up with a phone number that is already registered in the database, they should be shown a message directing them to sign in instead of proceeding with signup.

## Solution Implemented

### 1. Updated SignUpScreen (Android App)
**File**: `LivingAi_Lg/app/src/main/java/com/example/livingai_lg/ui/screens/auth/SignUpScreen.kt`

**Changes**:
- Added a check before requesting OTP to verify if the user already exists
- If user exists and is fully registered (has a name), shows a toast message: "This phone number is already registered. Please sign in instead."
- Automatically navigates to the sign-in screen
- If user doesn't exist or is in the middle of signup (no name), proceeds with normal signup flow

**Flow**:
1. User fills signup form and clicks "Sign Up"
2. App calls `checkUser()` API to verify if phone number is registered
3. If user exists → Show message and navigate to sign-in
4. If user doesn't exist → Proceed with OTP request and signup

### 2. Enhanced check-user Endpoint (Backend)
**File**: `farm-auth-service/src/routes/authRoutes.js`

**Changes**:
- Updated the `/auth/check-user` endpoint to check if user has a name (fully registered)
- Returns `user_exists: true` only if:
  - User exists in database
  - User has a name (not just created by verify-otp)
- Returns `user_exists: false` if:
  - User doesn't exist, OR
  - User exists but has no name (incomplete signup - allow them to continue)

**Logic**:
```javascript
// Check if user exists and has a name (fully registered)
const found = await db.query(
  `SELECT id, name FROM users 
   WHERE (phone_number = $1 OR phone_number = $2)
     AND deleted = FALSE`,
  phoneSearchParams
);

if (found.rows.length === 0) {
  // User not found - allow signup
  return { user_exists: false };
}

const user = found.rows[0];
const isFullyRegistered = user.name && user.name.trim() !== '';

if (isFullyRegistered) {
  // User is fully registered - should sign in
  return { user_exists: true, message: 'User is already registered. Please sign in instead.' };
} else {
  // User exists but incomplete - allow signup to continue
  return { user_exists: false };
}
```

## User Experience

### Scenario 1: New User
1. User enters phone number that doesn't exist in database
2. Clicks "Sign Up"
3. System checks → User doesn't exist
4. Proceeds with OTP request and signup flow ✅

### Scenario 2: Fully Registered User
1. User enters phone number that exists and has a name
2. Clicks "Sign Up"
3. System checks → User exists and is fully registered
4. Shows toast: "This phone number is already registered. Please sign in instead."
5. Automatically navigates to sign-in screen ✅

### Scenario 3: Incomplete Signup
1. User previously started signup (verify-otp created user but didn't complete)
2. User enters same phone number again
3. Clicks "Sign Up"
4. System checks → User exists but has no name
5. Proceeds with signup to complete registration ✅

## Benefits

1. **Prevents Duplicate Accounts**: Users can't create multiple accounts with the same phone number
2. **Better UX**: Clear message directing users to sign in if already registered
3. **Handles Edge Cases**: Users who started but didn't complete signup can still finish
4. **Automatic Navigation**: Seamlessly redirects to sign-in screen

## Testing

To test the implementation:

1. **Test New User Signup**:
   - Enter a new phone number
   - Should proceed with signup normally

2. **Test Existing User**:
   - Enter a phone number that's already registered
   - Should show message and navigate to sign-in

3. **Test Incomplete Signup**:
   - Start signup but don't complete (verify OTP but don't finish)
   - Try to sign up again with same number
   - Should allow completion of signup

## API Response Examples

### User Exists (Fully Registered)
```json
{
  "success": true,
  "message": "User is already registered. Please sign in instead.",
  "user_exists": true
}
```

### User Doesn't Exist or Incomplete
```json
{
  "success": false,
  "error": "USER_NOT_FOUND",
  "message": "User is not registered. Please sign up to create a new account.",
  "user_exists": false
}
```