Plugin classloading breaks on Resin 3.0.9: "Duplicate plugin key found"

A user reports getting the following error in Resin 3.0.9, despite only having one atlassian-jira-rpc-plugin-1.2.jar instance in WEB-INF/lib:

2005-01-08 14:47:23,993 ERROR com.atlassian.jira.ComponentManager :
Error occurred while starting component.
com.atlassian.plugin.PluginParseException: Duplicate plugin key found:
'com.atlassian.jira.ext.rpc'
at
com.atlassian.plugin.DefaultPluginManager.addPlugin(DefaultPluginManager
.java:42)
at
com.atlassian.plugin.DefaultPluginManager.init(DefaultPluginManager.java
:33)
at
com.atlassian.jira.plugin.JiraPluginManager.start(JiraPluginManager.java
:35)
at
com.atlassian.jira.ComponentManager.quickStart(ComponentManager.java:213
)
at
com.atlassian.jira.ComponentManager.start(ComponentManager.java:194)
at
com.atlassian.jira.upgrade.ConsistencyLauncher.contextInitialized(Consis
tencyLauncher.java:25)
at
com.caucho.server.webapp.Application.start(Application.java:1530)
at com.caucho.server.deploy.Entry.startImpl(Entry.java:457)
at
com.caucho.server.webapp.WebAppEntry.startImpl(WebAppEntry.java:85)
at
com.caucho.server.deploy.Entry.redeployIfModifiedImpl(Entry.java:416)
at
com.caucho.server.webapp.WebAppEntry.redeployIfModifiedImpl(WebAppEntry.
java:85)
at
com.caucho.server.deploy.Entry.createLazyInstance(Entry.java:340)
at
com.caucho.server.webapp.WebAppEntry.createLazyInstance(WebAppEntry.java
:85)
at
com.caucho.server.webapp.ApplicationContainer.getApplication(Application
Container.java:795)
at
com.caucho.server.webapp.ApplicationContainer.buildInvocation(Applicatio
nContainer.java:669)
at com.caucho.server.host.Host.buildInvocation(Host.java:515)
at
com.caucho.server.host.HostContainer.buildInvocation(HostContainer.java:
296)
at
com.caucho.server.resin.ServletServer.buildInvocation(ServletServer.java
:550)
at
com.caucho.server.dispatch.DispatchServer.buildInvocation(DispatchServer
.java:175)
at
com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:244)
at
com.caucho.server.port.TcpConnection.run(TcpConnection.java:327)
at com.caucho.util.ThreadPool.runTasks(ThreadPool.java:450)
at com.caucho.util.ThreadPool.run(ThreadPool.java:394)
at java.lang.Thread.run(Thread.java:534)

This does not affect the normal running of JIRA, but presumably the RPC functionality is broken.