Fatal error: Cannot declare class

[Balbo76]

Good evening.

I have tried imposter-plugin for composer but i think i'm not using it in the right way: i placed this code in composer.json file:

"extra": {
    "imposter": {
      "namespace": "My\\Vendor",
      "excludes": [ ]
    }
  }

And then i installed the imposter plugin via composer require.
Then i run composer install and all my classes get namespaced but when i try to use them im getting the errors:

Fatal error: Cannot declare class Ekr\Vendor\Monolog\Formatter\LineFormatter, because the name is already in use in /var/www/html/.../vendor/monolog/monolog/src/Monolog/Formatter/LineFormatter.php on line 0

Wich is my mistake?