- [x] I have searched the [issues](https://github.com/apache/incubator-dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/incubator-dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.0
* Operating System version: Windows7
* Java version: jdk8

### Steps to reproduce this issue

1. stop dubbo service
2. dubbo client set checked = true.  do get cudbbo client for times.
3. the size of list field [urls] in ReferenceConfig wil increase

Pls. provide [GitHub address] to reproduce this issue.
(project)[https://github.com/vio-lin/incubator-dubbo/tree/error-situation]  
run java file 
 incubator-dubbo\dubbo-demo\dubbo-demo-xml\dubbo-demo-xml-consumer\src\main\java\org\apache\dubbo\demo\consumer\Application.java
### Expected Result
the [urls] in ReferenceConfig will be updated.

### Actual Result
the avalidable registery urls be add to [urls] in ReferenceConfig

* <li>when the url is dubbo://224.5.6.7:1234/org.apache.dubbo.config.api.DemoService?application=dubbo-sample, then
     * the protocol is <b>DubboProtocol</b></li>
     * <p>
     * Actually，when the {@link ExtensionLoader} init the {@link Protocol} instants,it will automatically wraps two
     * layers, and eventually will get a <b>ProtocolFilterWrapper</b> or <b>ProtocolListenerWrapper</b>
     */
    private static final Protocol REF_PROTOCOL = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
    /**
     * The {@link Cluster}'s implementation with adaptive functionality, and actually it will get a {@link Cluster}'s
     * specific implementation who is wrapped with <b>MockClusterInvoker</b>
     */
    private static final Cluster CLUSTER = ExtensionLoader.getExtensionLoader(Cluster.class).getAdaptiveExtension();
    /**
     * A {@link ProxyFactory} implementation that will generate a reference service's proxy,the JavassistProxyFactory is
     * its default implementation
     */
    private static final ProxyFactory PROXY_FACTORY = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();
    /**
     * The url of the reference service
     */
    private final List<URL> urls = new ArrayList<URL>();
    /**
     * The interface name of the reference service
     */
    private String interfaceName;
    /**
     * The interface class of the reference service
     */
    private Class<?> interfaceClass;
    /**