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 +296,29 @@
   private function renderCommentPanel() {
     $viewer = $this->getViewer();
 
+    // Set placeholder in comment field if a duplicate task - T15749
+    $placeholder_text = '';
+    $object_phid = $this->getObjectPHID();
+    if (substr_compare($object_phid, 'PHID-TASK-', 0, 10) === 0) {
+      $query = id(new ManiphestTaskQuery())
+        ->setViewer(PhabricatorUser::getOmnipotentUser())
+        ->needSubscriberPHIDs(false)
+        ->needProjectPHIDs(false);
+      $query->withPHIDs(array($object_phid));
+      $task = $query->executeOne();
+      if ($task->getStatus() === ManiphestTaskStatus::STATUS_CLOSED_DUPLICATE) {
+        $placeholder_text = 'This task is closed as a duplicate. '.
+        'Comment here only if you think that this task is not a duplicate.';
+      }
+    }
+
     $remarkup_control = id(new PhabricatorRemarkupControl())
       ->setViewer($viewer)
       ->setID($this->getCommentID())
       ->addClass('phui-comment-fullwidth-control')
       ->addClass('phui-comment-textarea-control')
       ->setCanPin(true)
+      ->setPlaceholder($placeholder_text)
       ->setName('comment');
 
     $draft_comment = '';