The $ validate structure is a bit confused, you have two identical array keys (usually a message) under the required key. It should be:
public $validate = array(
'username' => array(
'required' => array(
'rule' => array('notEmpty'),
'message' => __('A Username is required', true),
),
'unique'=>array(
'rule' => 'isUnique',
'message' => __('This username has already been taken', true)
)
)
);
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