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DuplicateArrayKey

Emitted when an array has a key more than once

<?php

$arr = [
    'a' => 'one',
    'b' => 'two',
    'c' => 'this text will be overwritten by the next line',
    'c' => 'three',
];

This can be caused by variadic arguments if @no-named-arguments is not specified:

<?php
function foo($bar, ...$baz): array
{
    return [$bar, ...$baz]; // $baz is array<array-key, mixed> since it can have named arguments
}

How to fix

Remove the offending duplicates:

<?php

$arr = [
    'a' => 'one',
    'b' => 'two',
    'c' => 'three',
];

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