ENGCOM-3431: Fixed subscribe to newsletter if you already have an account issue #8952 -2.3 develop #19164

 - Merge Pull Request #19164 from ravi-chandra3197/magento2:patch-8952-2.3-develop
 - Merged commits:
   1. 78b694f
   2. ebfffe4
   3. b8941a8

Author: magento-engcom-team

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

@@ -80,8 +80,9 @@ public function __construct(
     protected function validateEmailAvailable($email)
     {
         $websiteId = $this->_storeManager->getStore()->getWebsiteId();
-        if ($this->_customerSession->getCustomerDataObject()->getEmail() !== $email
-            && !$this->customerAccountManagement->isEmailAvailable($email, $websiteId)
+        if ($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

@@ -57,7 +57,7 @@ public function testNewActionUsedEmail()
         $this->dispatch('newsletter/subscriber/new');
 
         $this->assertSessionMessages($this->equalTo([
-                'This email address is already assigned to another user.',
+                'Thank you for your subscription.',
             ]));
         $this->assertRedirect($this->anything());
     }