Missing synchronization in PackageClassLoader causes a LinkageError: duplicate class definition

Creating multiple KieSessions from a KieBase with a function and using them concurrently from different threads may lead to the following error

java.util.concurrent.ExecutionException: java.lang.LinkageError: loader (instance of org/drools/core/rule/JavaDialectRuntimeData$PackageClassLoader): attempted duplicate class definition for name: "defaultpkg/IsEven"
at java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.util.concurrent.FutureTask.get(FutureTask.java:188)
at org.drools.compiler.integrationtests.Misc2Test.testFunctionInvokingFunction(Misc2Test.java:8191)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.junit.runner.JUnitCore.run(JUnitCore.java:160)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:78)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:212)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:68)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.LinkageError: loader (instance of org/drools/core/rule/JavaDialectRuntimeData$PackageClassLoader): attempted duplicate class definition for name: "defaultpkg/IsEven"
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
at org.drools.core.rule.JavaDialectRuntimeData$PackageClassLoader.fastFindClass(JavaDialectRuntimeData.java:673)
at org.drools.core.rule.JavaDialectRuntimeData$PackageClassLoader.loadClass(JavaDialectRuntimeData.java:639)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
at org.drools.core.rule.MVELDialectRuntimeData.getParserConfiguration(MVELDialectRuntimeData.java:299)
at org.drools.core.base.mvel.MVELCompilationUnit.getCompiledExpression(MVELCompilationUnit.java:214)
at org.drools.core.rule.constraint.MvelConstraint.createMvelConditionEvaluator(MvelConstraint.java:254)
at org.drools.core.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:238)
at org.drools.core.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:204)
at org.drools.core.reteoo.AlphaNode.assertObject(AlphaNode.java:141)
at org.drools.core.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleObjectSinkAdapter.java:60)
at org.drools.core.reteoo.ObjectTypeNode.propagateAssert(ObjectTypeNode.java:298)
at org.drools.core.phreak.PropagationEntry$Insert.execute(PropagationEntry.java:93)
at org.drools.core.phreak.SynchronizedPropagationList.flush(SynchronizedPropagationList.java:96)
at org.drools.core.phreak.SynchronizedPropagationList.flush(SynchronizedPropagationList.java:69)
at org.drools.core.impl.StatefulKnowledgeSessionImpl.flushPropagations(StatefulKnowledgeSessionImpl.java:1993)
at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1289)
at org.drools.core.impl.StatefulKnowledgeSessionImpl.internalFireAllRules(StatefulKnowledgeSessionImpl.java:1294)
at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1281)
at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1260)
at org.drools.compiler.integrationtests.Misc2Test$11.call(Misc2Test.java:8180)
at org.drools.compiler.integrationtests.Misc2Test$11.call(Misc2Test.java:8170)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)