diff --git a/src/infrastructure/daemon/PhabricatorDaemon.php b/src/infrastructure/daemon/PhabricatorDaemon.php
--- a/src/infrastructure/daemon/PhabricatorDaemon.php
+++ b/src/infrastructure/daemon/PhabricatorDaemon.php
@@ -34,6 +34,24 @@
       return $command;
     }
 
+    // We may reach this method while already running as the daemon user: for
+    // example, active and passive synchronization of clustered repositories
+    // run the same commands through the same code, but as different users.
+
+    // By default, `sudo` won't let you sudo to yourself, so we can get into
+    // trouble if we're already running as the daemon user unless the host has
+    // been configured to let the daemon user run commands as itself.
+
+    // Since this is silly and more complicated than doing this check, don't
+    // use `sudo` if we're already running as the correct user.
+    if (function_exists('posix_getuid')) {
+      $uid = posix_getuid();
+      $info = posix_getpwuid($uid);
+      if ($info && $info['name'] == $user) {
+        return $command;
+      }
+    }
+
     // Get the absolute path so we're safe against the caller wiping out
     // PATH.
     $sudo = Filesystem::resolveBinary('sudo');