Fix duplicate for existing classes

[before]
- classes that already exists are added when use cappua  code template

e.g. In case of all classes are AppController and ClassRegistry:
App::uses('AppController', 'Controller');
App::uses('ClassRegistry', 'Utility');
cappua

->
App::uses('AppController', 'Controller');
App::uses('ClassRegistry', 'Utility');
App::uses('AppController', 'Controller');
App::uses('ClassRegistry', 'Utility');

[after]
App::uses('AppController', 'Controller');
App::uses('ClassRegistry', 'Utility');
cappua

->
App::uses('AppController', 'Controller');
App::uses('ClassRegistry', 'Utility');
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