When JVM_AddModulePackage() and JVM_DefineModule() are called to add a package that is already in a module defined for the specified class loader then the JVM generates an IllegalArgumentError with the message:

    "Package %s [for module %s] already exists for class loader"

A more accurate message would be:

    "Package %s [for module %s] is already in another module defined to the class loader"

Additionally, the JVM should throw IllegalStateException for this issue, not IllegalArgumentException