Configuration of SpringJUnit4ClassRunner Test clashes with SpringBootTest

I have a bunch of tests in my project that are all annotated with @SpringBootTest and therefore load up a SpringBoot context.

Now recently I refactored a Test in which I wanted a smaller scope (it´s about process coverage with camunda) to @RunWith(SpringJUnit4ClassRunner.class). Since this means that no context is loaded automatically I create some beans "manually" with a static inner class configuration. The entire test looks something like this:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = {
        ExternalConfiguration.class, MyTest.InternalConfiguration.class
})
public class MyTest{

    @Autowired
    private SomeBean someInternalBean;

    @Configuration
    public static class InternalConfiguration{

        @Bean
        SomeBean someInternalBean() {
            return mock(SomeBean .class);
        }

    }
    //Tests

Now, this test runs fine when I run it. BUT when I run any other test ( those still annotated with @SpringBootTest), I get issues with when the ApplicationContext is loaded:

The bean 'someInternalBean', defined in class path resource [.../MyTest$InternalConfiguration.class], could not be registered. A bean with that name has already been defined in file [.../SomeBean.class] and overriding is disabled.

Apparently a bean is created when loading the ApplicationContext because the class is annotated with @Component AND the context loader tries to create another bean from my internal configuration.

I´m hesitant to allow bean-overriding because I´m worried my mock beans might overwrite the automatically created beans.

How do I circumvent this? I want my SpringJUnit4ClassRunner-tests with their internal configurations to not affect my other @SpringBootTest-tests.