diff --git a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
--- a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
+++ b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
@@ -134,6 +134,9 @@
           $add_slug_template = id(new PhabricatorProjectSlug())
             ->setProjectPHID($object->getPHID());
           foreach ($add as $add_slug_str) {
+            if ($add_slug_str === $object->getPrimarySlug()) {
+              continue;
+            }
             $add_slug = id(clone $add_slug_template)
               ->setSlug($add_slug_str)
               ->save();