<?php


$keywords['data'] = [
	'a_key' => 'a_keyword',
	'b_key' => 'b_keyword',
	'c_key' => 'c_keyword',
];

$result = array();
foreach ($keywords['data'] as $key => $one) {
  $result['data']['main'][$key] = $one;
  $result['data']['secondary'][$key] = $one;
}

var_export($result);