LinkageError when obtaining a Cipher instance in a multithreaded environment

When obtaining a Cipher instance the Groovy rootLoader can throw a LinkageError in relation to the provider implementation classes if multiple threads try get such a Cipher instance.

No unit test as I can't reliably reproduce the effect...
An example script to illustrate the issue (may need to run multiple times):

import javax.crypto.Cipher
import org.bouncycastle.jce.provider.BouncyCastleProvider
import java.security.Security

Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider())

def getCipherInstance = {Cipher.getInstance("Blowfish/ECB/PKCS5Padding", BouncyCastleProvider.PROVIDER_NAME)}

new Thread(getCipherInstance).start()
new Thread(getCipherInstance).start()

And the exception:

Exception in thread "Thread-2" org.codehaus.groovy.runtime.InvokerInvocationException: java.lang.LinkageError: loader (instance of org/codehaus/groovy/tools/RootLoader): attempted duplicate class definition for name: "org/bouncycastle/jce/provider/JCEBlockCipher$Blowfish"
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:92)
at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:234)
at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:272)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:893)
at groovy.lang.Closure.call(Closure.java:279)
at groovy.lang.Closure.call(Closure.java:274)
at groovy.lang.Closure.run(Closure.java:355)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.LinkageError: loader (instance of org/codehaus/groovy/tools/RootLoader): attempted duplicate class definition for name: "org/bouncycastle/jce/provider/JCEBlockCipher$Blowfish"
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$000(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at org.codehaus.groovy.tools.RootLoader.oldFindClass(RootLoader.java:152)
at org.codehaus.groovy.tools.RootLoader.loadClass(RootLoader.java:124)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.security.Provider$Service.getImplClass(Provider.java:1262)
at java.security.Provider$Service.newInstance(Provider.java:1220)
at javax.crypto.Cipher.getInstance(DashoA13*..)
at javax.crypto.Cipher.getInstance(DashoA13*..)
at javax.crypto.Cipher$getInstance$0.call(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:43)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:117)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:129)
at CipherTest$_run_closure1.doCall(CipherTest.groovy:7)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:86)
at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:234)
at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:272)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:893)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.callCurrent(PogoMetaClassSite.java:66)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:151)
at CipherTest$_run_closure1.doCall(CipherTest.groovy)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:86)
... 7 more