InstanceAlreadyExistsException for EnvironmentManager when trying to start Spring Boot application in plain Spring test

Question

I have simple web application:

@SpringBootApplication
public class Application {
   public static void main(String[] args) {
      SpringApplication.run(Application.class, args);
   }
}

@RestController
public class SimpleController {

   @Autowired
   private String sayHello;

   @GetMapping("/hello")
   public String hello() {
       return sayHello;
   }
}

@Configuration
public class StringConfig {
   @Bean
   public String sayHelloWorld() {
      return "Hello, World!";
   }
}

And add tests:

 //This is abstarct class for test.
 @WebAppConfiguration
 @RunWith(SpringRunner.class)
 @ContextConfiguration(classes = Application.class)
 public abstract class AbstractTestClass {

     @Autowired
     private WebApplicationContext webApplicationContext;

     protected MockMvc mockMvc;

     @Before
     public void init() {
        mockMvc = MockMvcBuilders.webAppContextSetup(webApplicationContext).build();
    }
 }

 //This is custom configuration for bean "sayHello".
 @Configuration
 public class ConfigurationForTest {
    @Bean
    public String sayHello() {
       return "Hello, Test1!";
    }
 }

 //This test loads custom config with ApplicationContext.
 @ContextConfiguration(classes = ConfigurationForTest.class)
 public class SimpleControllerTest1 extends AbstractTestClass {

    //Checks that enpoind return expected reponse.
    @Test
    public void shouldReturnHelloTest1() throws Exception {
    mockMvc.perform(MockMvcRequestBuilders.get("/hello"))
            .andExpect(MockMvcResultMatchers.content().string("Hello, Test1!"));
   }
}

//This test don't loads custom configuration and works on default ApplicationContext.
public class SimpleControllerTest2 extends AbstractTestClass {

   //Checks that enpoind return expected reponse.
   @Test
   public void shouldReturnHelloWorld() throws Exception {
      mockMvc.perform(MockMvcRequestBuilders.get("/hello"))
             .andExpect(MockMvcResultMatchers.content()
                                             .string("Hello,World!"));
   }
}

This test fails with:

Failed to load ApplicationContext java.lang.IllegalStateException: Failed to load ApplicationContext

Caused by: org.springframework.jmx.export.UnableToRegisterMBeanException: Unable to register MBean [org.springframework.cloud.context.environment.EnvironmentManager@1ed864e4] with key 'environmentManager'; nested exception is javax.management.InstanceAlreadyExistsException: org.springframework.cloud.context.environment:name=environmentManager,type=EnvironmentManager

Caused by: javax.management.InstanceAlreadyExistsException: org.springframework.cloud.context.environment:name=environmentManager,type=EnvironmentManager

What is it caused by? It isn't clear for me. I know that i can add @DirtiesContext(classMode = DirtiesContext.ClassMode.AFTER_CLASS)or use other annotation @WebMvcTest, @SpringBootTest + @AutoConfigureMockMvc. But I want to know why is it happend and and how to load the whole application context dirrectly using this approach if my application will be more complex?

What is the difference between @ContextConfiguration(classes=Application.class) and @SpringBootTest. Why does this error not occur when i use @SpringBootTest? With @SpringBootTest ApplciationContext is loaded twice too, but tests pass successfully.