Avoid multiple tag creation

When a new tag is created but not yet persisted, it can be duplicated.
It could happen when multiple rules match the content and at least 2 of them should attach same new tag.

Fix #1528

src/Wallabag/CoreBundle/Entity/Entry.php

@@ -462,6 +462,14 @@ class Entry
             return;
         }
 
+        // check if tag already exist but has not yet be persisted
+        // it seems that the previous condition with `contains()` doesn't check that case
+        foreach ($this->tags as $existingTag) {
+            if ($existingTag->getUser() !== $tag->getUser() || $existingTag->getLabel() === $tag->getLabel()) {
+                return;
+            }
+        }
+
         $this->tags[] = $tag;
         $tag->addEntry($this);
     }