diff --git a/src/applications/maniphest/editor/ManiphestEditEngine.php b/src/applications/maniphest/editor/ManiphestEditEngine.php
--- a/src/applications/maniphest/editor/ManiphestEditEngine.php
+++ b/src/applications/maniphest/editor/ManiphestEditEngine.php
@@ -69,6 +69,29 @@
     return pht('Set Sail for Adventure');
   }
 
+  /**
+   * Set a placeholder text in the comment field.
+   *
+   * @param string PHID of the current object the comment field is
+   *   displayed in to allow defining rendering conditions
+   * @return string Placeholder text to display in comment field
+   */
+  public function getCommentFieldPlaceholderText($object_phid) {
+    $viewer = $this->getViewer();
+    $query = id(new ManiphestTaskQuery())
+      ->setViewer($viewer)
+      ->needSubscriberPHIDs(false)
+      ->needProjectPHIDs(false);
+    $query->withPHIDs(array($object_phid));
+    $task = $query->executeOne();
+
+    if ($task->getStatus() === ManiphestTaskStatus::STATUS_CLOSED_DUPLICATE) {
+      return pht('This task is closed as a duplicate. '.
+      'Only comment here if you think that this task is not a duplicate.');
+    }
+    return '';
+  }
+
   protected function getObjectViewURI($object) {
     return '/'.$object->getMonogram();
   }
diff --git a/src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php b/src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php
--- a/src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php
+++ b/src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php
@@ -296,12 +296,38 @@
   private function renderCommentPanel() {
     $viewer = $this->getViewer();
 
+    // Set a placeholder text in the comment field defined via
+    // getCommentFieldPlaceholderText() in the object's EditEngine class
+    $placeholder_text = '';
+    $object_phid = $this->getObjectPHID();
+
+    $phid_types = mpull(PhabricatorPHIDType::getAllInstalledTypes($viewer),
+                        null, 'getTypeConstant');
+    $object_type = idx($phid_types, substr($object_phid, 5, 4));
+    // Find matching EditEngine for our object via ApplicationClasses
+    if ($object_type) {
+      $app_class = $object_type->getPHIDTypeApplicationClass();
+
+      $engines = id(new PhabricatorEditEngineQuery())
+        ->setViewer($this->getViewer())
+        ->execute();
+      $engines = mpull($engines, null, 'getEngineApplicationClass');
+      $engine = idx($engines, $app_class);
+      $engine_class = get_class($engine);
+
+      if (method_exists($engine_class, 'getCommentFieldPlaceholderText')) {
+        $placeholder_text =
+          $engine->getCommentFieldPlaceholderText($object_phid);
+      }
+    }
+
     $remarkup_control = id(new PhabricatorRemarkupControl())
       ->setViewer($viewer)
       ->setID($this->getCommentID())
       ->addClass('phui-comment-fullwidth-control')
       ->addClass('phui-comment-textarea-control')
       ->setCanPin(true)
+      ->setPlaceholder($placeholder_text)
       ->setName('comment');
 
     $draft_comment = '';