Fix pre-existing bug in descriptor builds

php/src/Google/Protobuf/Internal/DescriptorPool.php

@@ -20,6 +20,7 @@ class DescriptorPool
     private static $pool;
     // Map from message names to sub-maps, which are maps from field numbers to
     // field descriptors.
+    private $unique_descs = [];
     private $class_to_desc = [];
     private $class_to_enum_desc = [];
     private $proto_to_class = [];
@@ -71,6 +72,8 @@ class DescriptorPool
     {
         $this->proto_to_class[$descriptor->getFullName()] =
             $descriptor->getClass();
+        $this->unique_descs[$descriptor->getFullName()] =
+            $descriptor;
         $this->class_to_desc[$descriptor->getClass()] = $descriptor;
         $this->class_to_desc[$descriptor->getLegacyClass()] = $descriptor;
         $this->class_to_desc[$descriptor->getPreviouslyUnreservedClass()] = $descriptor;
@@ -163,7 +166,7 @@ class DescriptorPool
 
     public function finish()
     {
-        foreach ($this->class_to_desc as $klass => $desc) {
+        foreach ($this->unique_descs as $klass => $desc) {
             $this->crossLink($desc);
         }
         unset($desc);