cakephp the rule validation function run twice

whene i use this code it's repeat the function isExif_imagetype

        'ProfileImage' => array(

           'rule'       => 'isExif_imagetype', // ou bien : array('nomRegle', 'parametre1', 'parametre2' ...)

         'message'    => 'Votre message d\'erreur'

           ),

, but whene i add         'on'         => 'create', 

 it's does't repeat.

and for example whene i debug i got twice message. how to make it's run without repeating ??

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