Add new entity action to check for existing post action.

app/bundles/FormBundle/Controller/Api/FormApiController.php

@@ -327,4 +327,16 @@ protected function createFieldEntityForm($entity)
             ]
         );
     }
+
+    public function newEntityAction(Request $request): Response
+    {
+        // Check for an email to see if the lead already exists
+        $parameters = $request->request->all();
+
+        if (!isset($parameters['postAction'])) {
+            $request->request->add(['postAction' => 'return']);
+        }
+
+        return parent::newEntityAction($request);
+    }
 }