Prevents repeat subscription confirmation

Prevents scheduling of duplicate welcome notifications

lib/Subscription/Pages.php

@@ -57,7 +57,7 @@ class Pages {
   }
 
   function confirm() {
-    if($this->subscriber === false) {
+    if($this->subscriber === false || $this->subscriber->status === Subscriber::STATUS_SUBSCRIBED) {
       return false;
     }
 
@@ -205,7 +205,6 @@ class Pages {
     }
   }
 
-
   private function getConfirmContent() {
     if($this->isPreview() || $this->subscriber !== false) {
       return __("Yup, we've added you to our email list. You'll hear from us shortly.", 'mailpoet');