diff --git a/src/applications/repository/daemon/PhabricatorGitGraphStream.php b/src/applications/repository/daemon/PhabricatorGitGraphStream.php
index 5cbac88a7e..9133983677 100644
--- a/src/applications/repository/daemon/PhabricatorGitGraphStream.php
+++ b/src/applications/repository/daemon/PhabricatorGitGraphStream.php
@@ -29,7 +29,12 @@ final class PhabricatorGitGraphStream
     if (!isset($this->parents[$commit])) {
       $this->parseUntil($commit);
     }
-    return $this->parents[$commit];
+    $parents = $this->parents[$commit];
+
+    // NOTE: In Git, it is possible for a commit to list the same parent more
+    // than once. See T5226. Discard duplicate parents.
+
+    return array_unique($parents);
   }
 
   public function getCommitDate($commit) {