- fix duplicate problem (#6981)

* - fix duplicate problem

* style guide adjustment

* replace tab with 4-space

* replace tab with 4 -space

modules/swagger-codegen/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache

@@ -268,6 +268,9 @@ public class ApiClient {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
+        if (defaultHeaders.containsKey(name)) {
+            defaultHeaders.remove(name);
+        }
         defaultHeaders.add(name, value);
         return this;
     }