ReferenceLettersBean rf = Mockito.mock(ReferenceLettersBean.class);
     rf.setBoolcheck(false);

Mockito.doNothing().when(rf).checkForDuplicates(anyString(),     anyString(), anyString());
         rf.checkForDuplicates("abcd@emal.com","abd@gmail.com","abcd@gmail.com");
assertEquals(true,rf.getBoolcheck());

SomeDependency dependency = mock(SomeDependency.class);

// Make a REAL ReferenceLettersBean
ReferenceLettersBean bean = new ReferenceLettersBean(dependency);

// now make your test
rf.checkForDuplicates("abcd@emal.com","abd@gmail.com","abcd@gmail.com");
assertEquals(true,rf.getBoolcheck());