Custom CLI Return "Cannot declare class name in use"

You wrote:
Should I use ROOTPATH instead of APPPATH?

NO, Use APPPATH if these folders are inside the app/ folder.

Edit your code file (make sure it is named PrintHelloWorld.php and lives in app/ThirdParty/trydummy/src/Commands)
change:
namespace someone\plugin\Commands;
to:
namespace someone\plugins\Commands; (you left the 's' off of plugins - it therefore did not match your Autoload.php entry)

I've tested it and it works.