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 +69,28 @@
     return pht('Set Sail for Adventure');
   }
 
+  /**
+   * Set a placeholder text in the Maniphest task comment field.
+   *
+   * @param string PHID of the task the comment field is displayed in
+   * @return string Placeholder text to display
+   */
+  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 if you think that this task is not a duplicate.');
+    }
+    return '';
+  }
+
   protected function getObjectViewURI($object) {
     return '/'.$object->getMonogram();
   }
diff --git a/src/applications/transactions/editengine/PhabricatorEditEngine.php b/src/applications/transactions/editengine/PhabricatorEditEngine.php
--- a/src/applications/transactions/editengine/PhabricatorEditEngine.php
+++ b/src/applications/transactions/editengine/PhabricatorEditEngine.php
@@ -1668,6 +1668,7 @@
       ->setHeaderText($header_text)
       ->setAction($comment_uri)
       ->setRequiresMFA($requires_mfa)
+      ->setEditEngine($this)
       ->setSubmitButtonName($button_text);
 
     $draft = PhabricatorVersionedDraft::loadDraft(
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
@@ -20,6 +20,7 @@
   private $noPermission;
   private $fullWidth;
   private $infoView;
+  private $editEngine;
   private $editEngineLock;
   private $noBorder;
   private $requiresMFA;
@@ -150,6 +151,15 @@
     return $this->noPermission;
   }
 
+  public function setEditEngine(PhabricatorEditEngine $edit_engine) {
+    $this->editEngine = $edit_engine;
+    return $this;
+  }
+
+  public function getEditEngine() {
+    return $this->editEngine;
+  }
+
   public function setEditEngineLock(PhabricatorEditEngineLock $lock) {
     $this->editEngineLock = $lock;
     return $this;
@@ -296,12 +306,22 @@
   private function renderCommentPanel() {
     $viewer = $this->getViewer();
 
+    // Placeholder text in the <textarea> comment field. Defined in the
+    // corresponding EditEngine class via getCommentFieldPlaceholderText()
+    $engine = $this->getEditEngine();
+    $placeholder_text = '';
+    if ($engine && method_exists($engine, 'getCommentFieldPlaceholderText')) {
+      $placeholder_text = $engine
+        ->getCommentFieldPlaceholderText($this->getObjectPHID());
+    }
+
     $remarkup_control = id(new PhabricatorRemarkupControl())
       ->setViewer($viewer)
       ->setID($this->getCommentID())
       ->addClass('phui-comment-fullwidth-control')
       ->addClass('phui-comment-textarea-control')
       ->setCanPin(true)
+      ->setPlaceholder($placeholder_text)
       ->setName('comment');
 
     $draft_comment = '';