Error log when importing twice a XAR that happen to be an extension

2012-11-13 16:13:48.309:INFO::Server started, you can now open http://localhost:8080/ in your browser to access your wiki.
2012-11-13 16:13:48.309:INFO::----------------------------------
2012-11-13 16:15:44,902 [http://localhost:8080/xwiki/bin/view/Main/] INFO  .HibernateDataMigrationManager - Running storage schema updates and migrations 
2012-11-13 16:15:44,948 [http://localhost:8080/xwiki/bin/view/Main/] INFO  .HibernateDataMigrationManager - Starting migration for database  [xwiki]... 
2012-11-13 16:15:44,948 [http://localhost:8080/xwiki/bin/view/Main/] INFO  .HibernateDataMigrationManager - No storage migration required since current version is [43000] 
2012-11-13 16:19:11,941 [http://localhost:8080/xwiki/bin/login/XWiki/XWikiLogin] WARN  o.x.v.i.DefaultVelocityEngine  - Deprecated usage of method [com.xpn.xwiki.api.XWiki.parseMessage] in /templates/login.vm@29,33 
2012-11-13 16:19:16,917 [http://localhost:8080/xwiki/bin/loginsubmit/XWiki/XWikiLogin] WARN  o.x.v.i.DefaultVelocityEngine  - Deprecated usage of method [com.xpn.xwiki.api.XWiki.parseMessage] in /templates/login.vm@29,33 
2012-11-13 16:20:11,252 [http://localhost:8080/xwiki/bin/import/XWiki/XWikiPreferences?editor=globaladmin&section=Import] WARN  o.h.d.f.TemplateRenderer       - Function template anticipated 4 arguments, but 3 arguments encountered 
2012-11-13 16:20:11,252 [http://localhost:8080/xwiki/bin/import/XWiki/XWikiPreferences?editor=globaladmin&section=Import] WARN  o.h.d.f.TemplateRenderer       - Function template anticipated 4 arguments, but 3 arguments encountered 
2012-11-13 16:20:41,770 [http://localhost:8080/xwiki/bin/import/XWiki/XWikiPreferences?editor=globaladmin&section=Import] ERROR c.x.x.p.p.Package              - Failed to register extenion [org.xwiki.enterprise:xwiki-enterprise-ui-all-4.3-milestone-1] from the XAR 
org.xwiki.extension.InstallException: The extension [org.xwiki.enterprise:xwiki-enterprise-ui-all-4.3-milestone-1] is already installed on namespace [wiki:xwiki]
	at org.xwiki.extension.repository.internal.installed.DefaultInstalledExtensionRepository.installExtension(DefaultInstalledExtensionRepository.java:559) ~[xwiki-commons-extension-api-4.3-milestone-1.jar:na]
	at com.xpn.xwiki.plugin.packaging.Package.registerExtension(Package.java:709) [xwiki-platform-legacy-oldcore-4.3-milestone-1.jar:na]
	at com.xpn.xwiki.plugin.packaging.Package.install(Package.java:675) [xwiki-platform-legacy-oldcore-4.3-milestone-1.jar:na]
	at com.xpn.xwiki.plugin.packaging.PackageAPI.install(PackageAPI.java:316) [xwiki-platform-legacy-oldcore-4.3-milestone-1.jar:na]
	at com.xpn.xwiki.web.ImportAction.render(ImportAction.java:132) [xwiki-platform-legacy-oldcore-4.3-milestone-1.jar:na]
	at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:231) [xwiki-platform-legacy-oldcore-4.3-milestone-1.jar:na]
	at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:116) [xwiki-platform-legacy-oldcore-4.3-milestone-1.jar:na]
	at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431) [struts-1.2.9.jar:1.2.9]
	at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236) [struts-1.2.9.jar:1.2.9]
	at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196) [struts-1.2.9.jar:1.2.9]
	at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432) [struts-1.2.9.jar:1.2.9]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:727) [servlet-api-2.5.jar:2.5]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:820) [servlet-api-2.5.jar:2.5]
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:538) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1352) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:120) [xwiki-platform-legacy-oldcore-4.3-milestone-1.jar:na]
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1323) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.xwiki.wysiwyg.server.filter.ConversionFilter.doFilter(ConversionFilter.java:144) [xwiki-platform-wysiwyg-server-4.3-milestone-1.jar:na]
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1323) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at com.xpn.xwiki.plugin.webdav.XWikiDavFilter.doFilter(XWikiDavFilter.java:66) [xwiki-platform-webdav-server-4.3-milestone-1.jar:na]
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1323) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFilter(SavedRequestRestorerFilter.java:208) [xwiki-platform-container-servlet-4.3-milestone-1.jar:na]
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1323) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:111) [xwiki-platform-container-servlet-4.3-milestone-1.jar:na]
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1323) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:476) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:119) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:517) [jetty-security-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:225) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:937) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:406) [jetty-servlet-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:183) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:871) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:117) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:247) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:149) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:110) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.Server.handle(Server.java:346) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.java:589) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.HttpConnection$RequestHandler.content(HttpConnection.java:1065) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:823) [jetty-http-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:220) [jetty-http-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.server.HttpConnection.handle(HttpConnection.java:411) [jetty-server-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:535) [jetty-io-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:40) [jetty-io-7.4.5.v20110725.jar:7.4.5.v20110725]
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:529) [jetty-util-7.4.5.v20110725.jar:7.4.5.v20110725]
	at java.lang.Thread.run(Thread.java:679) [na:1.6.0_22]