<?php
$array = array(1, "hello", 1, "world", "hello");
print_r(array_count_values($array));
?>

Array
(
    [1] => 2
    [hello] => 2
    [world] => 1
)

<?php
$array = array(1, "hello", 1, "world", "hello");
$array_count = array_count_values($array);
$duplicates = [];
foreach ($array_count as $k=>$v){
    if ($v > 1) $duplicates[] = $k;
}
print_r($duplicates);
?>

array = [ 
      {
        'header':'header1',
        'contents':'contents1'
      },
      {
        'header':'header2',
        'contents':'contents2'
      },
    ....
];