feat: prevent duplicate membership applications and fees

- Add UNIQUE constraint on membership_application.user_id (one app per user)
- Add UNIQUE constraint on membership_fees.user_id (one fee record per user)
- Add validation checks in process_application.php before inserting
- Improve error messages for duplicate submission attempts
- Add migration script to clean up existing duplicates before constraints
- Update checkMembershipApplication to set session message on redirect
- Add comprehensive documentation of duplicate prevention architecture

Individual payments/EFTs are tracked separately in payments table

src/config/functions.php

@@ -1434,6 +1434,10 @@ function checkMembershipApplication($user_id)
 
     // Check if the record exists and redirect
     if ($count > 0) {
+        // Set a session message before redirecting
+        if (!isset($_SESSION['message'])) {
+            $_SESSION['message'] = 'You have already submitted a membership application.';
+        }
         header("Location: membership_details.php");
         exit();
     }

src/processors/process_application.php

@@ -18,6 +18,40 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
         die('Security token validation failed. Please try again.');
     }
 
+    // Check if user already has a membership application
+    $check_stmt = $conn->prepare("SELECT COUNT(*) as count FROM membership_application WHERE user_id = ?");
+    $check_stmt->bind_param("i", $user_id);
+    $check_stmt->execute();
+    $check_result = $check_stmt->get_result();
+    $check_row = $check_result->fetch_assoc();
+    $check_stmt->close();
+
+    if ($check_row['count'] > 0) {
+        http_response_code(400);
+        echo json_encode([
+            'status' => 'error',
+            'message' => 'You have already submitted a membership application. Please check your email for membership details.'
+        ]);
+        exit;
+    }
+
+    // Check if user already has a membership fee record
+    $fee_check_stmt = $conn->prepare("SELECT COUNT(*) as count FROM membership_fees WHERE user_id = ?");
+    $fee_check_stmt->bind_param("i", $user_id);
+    $fee_check_stmt->execute();
+    $fee_result = $fee_check_stmt->get_result();
+    $fee_row = $fee_result->fetch_assoc();
+    $fee_check_stmt->close();
+
+    if ($fee_row['count'] > 0) {
+        http_response_code(400);
+        echo json_encode([
+            'status' => 'error',
+            'message' => 'You already have a membership fee record. Please contact support if you need to update your application.'
+        ]);
+        exit;
+    }
+
     // Get all the form fields with validation
     $first_name = validateName($_POST['first_name'] ?? '');
     if ($first_name === false) {
@@ -188,11 +222,20 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
         // Rollback the transaction in case of error
         $conn->rollback();
 
-        // Error response
-        $response = [
-            'status' => 'error',
-            'message' => 'Error: ' . $e->getMessage()
-        ];
+        // Check for duplicate key error
+        $errorMessage = $e->getMessage();
+        if (strpos($errorMessage, 'Duplicate') !== false || strpos($errorMessage, '1062') !== false) {
+            $response = [
+                'status' => 'error',
+                'message' => 'You have already submitted a membership application. Please check your email for membership details.'
+            ];
+        } else {
+            // Error response
+            $response = [
+                'status' => 'error',
+                'message' => 'Error: ' . $errorMessage
+            ];
+        }
     }
 
     // Return the response in JSON format