diff --git a/src/aphront/configuration/AphrontApplicationConfiguration.php b/src/aphront/configuration/AphrontApplicationConfiguration.php
--- a/src/aphront/configuration/AphrontApplicationConfiguration.php
+++ b/src/aphront/configuration/AphrontApplicationConfiguration.php
@@ -772,6 +772,11 @@
         $multipart_parser->continueParse($raw_input);
         $parts = $multipart_parser->endParse();
 
+        // We're building and then parsing a query string so that requests
+        // with arrays (like "x[]=apple&x[]=banana") work correctly. This also
+        // means we can't use "phutil_build_http_querystring()", since it
+        // can't build a query string with duplicate names.
+
         $query_string = array();
         foreach ($parts as $part) {
           if (!$part->isVariable()) {
@@ -780,8 +785,7 @@
 
           $name = $part->getName();
           $value = $part->getVariableValue();
-
-          $query_string[] = urlencode($name).'='.urlencode($value);
+          $query_string[] = rawurlencode($name).'='.rawurlencode($value);
         }
         $query_string = implode('&', $query_string);
         $post = $parser->parseQueryString($query_string);
diff --git a/src/applications/auth/factor/PhabricatorDuoAuthFactor.php b/src/applications/auth/factor/PhabricatorDuoAuthFactor.php
--- a/src/applications/auth/factor/PhabricatorDuoAuthFactor.php
+++ b/src/applications/auth/factor/PhabricatorDuoAuthFactor.php
@@ -504,10 +504,7 @@
     $push_info = array(
       pht('Domain') => $this->getInstallDisplayName(),
     );
-    foreach ($push_info as $k => $v) {
-      $push_info[$k] = rawurlencode($k).'='.rawurlencode($v);
-    }
-    $push_info = implode('&', $push_info);
+    $push_info = phutil_build_http_querystring($push_info);
 
     $parameters = array(
       'username' => $duo_user,
diff --git a/src/applications/auth/future/PhabricatorDuoFuture.php b/src/applications/auth/future/PhabricatorDuoFuture.php
--- a/src/applications/auth/future/PhabricatorDuoFuture.php
+++ b/src/applications/auth/future/PhabricatorDuoFuture.php
@@ -91,11 +91,7 @@
       $http_method = $this->getHTTPMethod();
 
       ksort($data);
-      $data_parts = array();
-      foreach ($data as $key => $value) {
-        $data_parts[] = rawurlencode($key).'='.rawurlencode($value);
-      }
-      $data_parts = implode('&', $data_parts);
+      $data_parts = phutil_build_http_querystring($data);
 
       $corpus = array(
         $date,
diff --git a/src/applications/diffusion/controller/DiffusionServeController.php b/src/applications/diffusion/controller/DiffusionServeController.php
--- a/src/applications/diffusion/controller/DiffusionServeController.php
+++ b/src/applications/diffusion/controller/DiffusionServeController.php
@@ -528,7 +528,7 @@
         unset($query_data[$key]);
       }
     }
-    $query_string = http_build_query($query_data, '', '&');
+    $query_string = phutil_build_http_querystring($query_data);
 
     // We're about to wipe out PATH with the rest of the environment, so
     // resolve the binary first.