Bean type class com.opensymphony.xwork2.ObjectFactory with the name xwork has already been loaded by bean - struts2

Question

I'm trying to deploy a simple webapp with an embedded instance of Jetty. My embedded test code is:
public static void main(String[] args) throws Exception{
Server server = new Server(8181);
WebAppContext webAppContext = new WebAppContext();
webAppContext.setContextPath("/");
webAppContext.setWar(ContUtil.warPath + ContUtil.warName);
server.setHandler(webAppContext);
logger.info("starting jetty...");
server.start();
server.join();
}
but I got the following error:
Caused by: Unable to load bean: type: class:com.opensymphony.xwork2.ObjectFactory - bean - jar:file:/C:/Users/lingguo/AppData/Local/Temp/jetty-0.0.0.0-8181-MemCloud-Web-1.0-SNAPSHOT.war-_-any-6830790104248388668.dir/webapp/WEB-INF/lib/struts2-core-2.2.1.jar!/struts-default.xml:29:72
at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.register(XmlConfigurationProvider.java:232)
at org.apache.struts2.config.StrutsXmlConfigurationProvider.register(StrutsXmlConfigurationProvider.java:101)
at com.opensymphony.xwork2.config.impl.DefaultConfiguration.reloadContainer(DefaultConfiguration.java:180)
at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:66)
... 23 more
Caused by: Bean type class com.opensymphony.xwork2.ObjectFactory with the name xwork has already been loaded by bean - jar:file:/D:/Program%20Files%20(x86)/apache-maven-2.2.1/repo/org/apache/struts/struts2-core/2.2.1/struts2-core-2.2.1.jar!/struts-default.xml:29:72 - bean - jar:file:/C:/Users/lingguo/AppData/Local/Temp/jetty-0.0.0.0-8181-MemCloud-Web-1.0-SNAPSHOT.war-_-any-6830790104248388668.dir/webapp/WEB-INF/lib/struts2-core-2.2.1.jar!/struts-default.xml:29:72
at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.register(XmlConfigurationProvider.java:216)
... 26 more
I've referenced many similar answers about this problems, for example: Struts2 Error when Deploying: Unable to load bean: type: class:com.opensymphony.xwork2.ObjectFactory; Most of the similar problems are caused by jar packages conflict of different versions. But my problem is a little different. I ensure that there is only one version of struts2-core (2.2.1) in my classpath.
I checked the error message, I think that jetty creates a tmp file, that is: /C:/Users/lingguo/AppData/Local/Temp/jetty-0.0.0.0-8181-MemCloud-Web-1.0-SNAPSHOT.war-_-any-6830790104248388668.dir，and the jar struts2-core-2.2.1 is confict with the jar in my local maven repository.
Could anybody know where is going wrong? Thanks in advance!