Custom CLI Return "Cannot declare class name in use"

Assuming your app/config/Autoload.php contains this entry:
'Someone\plugins' => ROOTPATH . 'someone/plugins'

Then the namespace in your code says that your file is in:
someone/plugins/Commands folder
I've tested the above and your code works.

I don't understand your Edit: line  "I put it on the ThirdParty folder. Full path is on "ThirdParty\trydummy\src".
Your file should not be in this directory, it has to be in a 'Commands' folder

Try using php spark namespaces to check your namespaces map and can be found.

Once you have it working you could change the namespace mapping:
Eg: In app/config/Autoload.php
CustomSpark => ROOTPATH . 'someone/plugins'
and in your file:
namespace CustomSpark\Commands;

OR something even more sensible !