Duplicate application of the same mockup class

Question

When does the below error

java.lang.IllegalStateException: Duplicate application of the same mock-up class_ exception

occurs?

@Before
  public void setUp() throws Exception {
    new MockUp<URL>() {
      @Mock
      public void $init(String location) {}
    }.getMockInstance();
    new MockUp<CommunicationUtils>() {
      @Mock
      public void $clinit() {}
    }.getMockInstance();
  }

At line number 7, I am getting this exception.