<?php
 
$array = array(
    'item-1' => array(
        'a' => 'Повторяющееся значение 1',
        'b' => 'xx1',
        'c' => 'xx2'
    ),
    'item-2' => array(
        'a' => 'Повторяющееся значение 1',
        'b' => 'xx3',
        'c' => 'xx4'
    ),
    'item-3' => array(
        'a' => 'Повторяющееся значение 2',
        'b' => 'xx5',
        'c' => 'xx6'
    ),
    'item-4' => array(
        'a' => 'Повторяющееся значение 2',
        'b' => 'xx7',
        'c' => 'xx8'
    )
);
 
foreach (array_reverse($array, true) as $id => $item) {
    if (in_array($item['a'], array_column(array_slice($array, $id + 1), 'a'))) {
        unset($array[$id]);
    }
}
 
var_dump($array);

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