"Duplicate class definiton" error with interfaces

In file bug16Interface.groovy put:

interface bug16Interface { }

In file bug16.groovy put:

class bug16 implements bug16Interface {}

Typing "groovy -d bug16.groovy" gives:

$ groovy -d bug16.groovy
Caught: java.lang.LinkageError: duplicate class definition: bug16Interface
java.lang.LinkageError: duplicate class definition: bug16Interface
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at groovy.lang.GroovyClassLoader.access$300(GroovyClassLoader.java:92)
at groovy.lang.GroovyClassLoader$ClassCollector.createClass(GroovyClassLoader.java:457)
at groovy.lang.GroovyClassLoader$ClassCollector.onClassNode(GroovyClassLoader.java:475)
at groovy.lang.GroovyClassLoader$ClassCollector.call(GroovyClassLoader.java:479)
at org.codehaus.groovy.control.CompilationUnit$9.call(CompilationUnit.java:757)
at org.codehaus.groovy.control.CompilationUnit.applyToPrimaryClassNodes(CompilationUnit.java:932)
at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:478)
at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:306)
at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:275)
at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:221)
at groovy.lang.GroovyShell$2.run(GroovyShell.java:230)
at java.security.AccessController.doPrivileged(Native Method)
at groovy.lang.GroovyShell.run(GroovyShell.java:228)
at groovy.lang.GroovyShell.run(GroovyShell.java:173)
at groovy.ui.GroovyMain.processOnce(GroovyMain.java:483)
at groovy.ui.GroovyMain.run(GroovyMain.java:293)
at groovy.ui.GroovyMain.process(GroovyMain.java:279)
at groovy.ui.GroovyMain.main(GroovyMain.java:145)
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:585)
at org.codehaus.groovy.tools.GroovyStarter.rootLoader(GroovyStarter.java:131)
at org.codehaus.groovy.tools.GroovyStarter.main(GroovyStarter.java:160)