Symfony-Test: The container is initialized twice

I am writing a test and trying to get dependencies through KernelTestCase.

I have my own register that fills through the configuration file when the container is initialized. However, when I run bootKernel, two containers (service container, test container) are initializing.

It ends with my exception that the extension in my register already exists. Because the extension was added at first initialization. Has anyone encountered a similar problem?

I use Symfony 4.3