diff --git a/src/Authentication/Authenticators/Session.php b/src/Authentication/Authenticators/Session.php
index ffc54517a..b6f4b6b6d 100644
--- a/src/Authentication/Authenticators/Session.php
+++ b/src/Authentication/Authenticators/Session.php
@@ -550,6 +550,20 @@ private function checkRememberMeToken(string $remember)
      */
     public function startLogin(User $user): void
     {
+        /** @var int|string|null $userId */
+        $userId = $this->getSessionKey('id');
+
+        // Check if already logged in.
+        if ($userId !== null) {
+            throw new LogicException(
+                'The user has User Info in Session, so already logged in or in pending login state.'
+                . ' If a logged in user logs in again with other account, the session data of the previous'
+                . ' user will be used as the new user.'
+                . ' Fix your code to prevent users from logging in without logging out or delete the session data.'
+                . ' user_id: ' . $userId
+            );
+        }
+
         $this->user = $user;
 
         // Regenerate the session ID to help protect against session fixation
diff --git a/tests/Authentication/Authenticators/SessionAuthenticatorTest.php b/tests/Authentication/Authenticators/SessionAuthenticatorTest.php
index 171a7e5cf..0c85ba70c 100644
--- a/tests/Authentication/Authenticators/SessionAuthenticatorTest.php
+++ b/tests/Authentication/Authenticators/SessionAuthenticatorTest.php
@@ -7,6 +7,7 @@
 use CodeIgniter\Shield\Authentication\Authenticators\Session;
 use CodeIgniter\Shield\Config\Auth;
 use CodeIgniter\Shield\Entities\User;
+use CodeIgniter\Shield\Exceptions\LogicException;
 use CodeIgniter\Shield\Models\RememberModel;
 use CodeIgniter\Shield\Models\UserModel;
 use CodeIgniter\Shield\Result;
@@ -340,6 +341,26 @@ public function testAttemptSuccess(): void
         ]);
     }
 
+    public function testAttemptUserHavingSessionDataAttemptsAgain(): void
+    {
+        $_SESSION['user']['id'] = '999';
+
+        $this->expectException(LogicException::class);
+        $this->expectExceptionMessage(
+            'The user has User Info in Session, so already logged in or in pending login state.'
+        );
+
+        $this->user->createEmailIdentity([
+            'email'    => 'foo@example.com',
+            'password' => 'secret123',
+        ]);
+
+        $this->auth->attempt([
+            'email'    => $this->user->email,
+            'password' => 'secret123',
+        ]);
+    }
+
     public function testAttemptCaseInsensitive(): void
     {
         $this->user->createEmailIdentity([