$array = ['abc', 'def', 'ghi', 'abc'];

if($array[0] == $array[1])
etc

<?php
$input = array("a" => "green", "red", "b" => "green", "blue", "red");
$result = array_unique($input);
print_r($result);
?>

Array
(
    [a] => green
    [0] => red
    [1] => blue
)