DuplicateArrayKey

Emitted when an array has a key more than once

<?php

$arr = [
    'a' => 1,
    'b' => 2,
    'c' => 3,
    'c' => 4,
];

How to fix

Remove the offending duplicates:

<?php

$arr = [
    'a' => 1,
    'b' => 2,
    'c' => 4,
];

The first matching 'c' key was removed to prevent a change in behaviour (any new duplicate keys overwrite the values of prevvious ones).