ENGCOM-3680: [Backport] Fixed subscribe to newsletter if you already have an account issue #18912

 - Merge Pull Request #18912 from ravi-chandra3197/magento2:patch-8952
 - Merged commits:
   1. b1fc4a7

Author: magento-engcom-team

app/code/Magento/Newsletter/Controller/Subscriber/NewAction.php

@@ -77,9 +77,10 @@ public function __construct(
     protected function validateEmailAvailable($email)
     {
         $websiteId = $this->_storeManager->getStore()->getWebsiteId();
-        if ($this->_customerSession->getCustomerDataObject()->getEmail() !== $email
+        if ($this->_customerSession->isLoggedIn()
+            && ($this->_customerSession->getCustomerDataObject()->getEmail() !== $email
             && !$this->customerAccountManagement->isEmailAvailable($email, $websiteId)
-        ) {
+        )) {
             throw new LocalizedException(
                 __('This email address is already assigned to another user.')
             );

dev/tests/integration/testsuite/Magento/Newsletter/Controller/SubscriberTest.php

@@ -53,11 +53,10 @@ public function testNewActionUsedEmail()
         $this->getRequest()->setPostValue([
             'email' => 'customer@example.com',
         ]);
-
         $this->dispatch('newsletter/subscriber/new');
 
         $this->assertSessionMessages($this->equalTo([
-                'There was a problem with the subscription: This email address is already assigned to another user.',
+                'Thank you for your subscription.',
             ]));
         $this->assertRedirect($this->anything());
     }