Undefined index when running in parallel with same paths defined in different styles

[func0der]

Describe the bug
If you define the same 'file' in two different notations, parallel running of phpcs triggers:

Code sample

<?php

namespace Func0der\PhpCsTest;

class Test
{

}

Custom ruleset

<?xml version="1.0"?>
<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" name="PHP_CodeSniffer" xsi:noNamespaceSchemaLocation="vendor/squizlabs/php_codesniffer/phpcs.xsd">
    <rule ref="PSR12"/>
    <file>src</file>
    <file>./src</file>
</ruleset>

To reproduce
Steps to reproduce the behavior:

1. Create a file called src/Test.php and a phpcs.xml with the code sample above
2. Run phpcs --parallel=50
3. See error message displayed

PHP Fatal error:  Uncaught PHP_CodeSniffer\Exceptions\RuntimeException: Undefined index:  in /home/vagrant/phpcs-test/vendor/squizlabs/php_codesniffer/src/Files/FileList.php on line 187 in /home/vagrant/phpcs-test/vendor/squizlabs/php_codesniffer/src/Runner.php:606
Stack trace:
#0 /home/vagrant/phpcs-test/vendor/squizlabs/php_codesniffer/src/Files/FileList.php(187): PHP_CodeSniffer\Runner->handleErrors()
#1 /home/vagrant/phpcs-test/vendor/squizlabs/php_codesniffer/src/Runner.php(487): PHP_CodeSniffer\Files\FileList->current()
#2 /home/vagrant/phpcs-test/vendor/squizlabs/php_codesniffer/src/Runner.php(114): PHP_CodeSniffer\Runner->run()
#3 /home/vagrant/phpcs-test/vendor/squizlabs/php_codesniffer/bin/phpcs(18): PHP_CodeSniffer\Runner->runPHPCS()
#4 {main}
  thrown in /home/vagrant/phpcs-test/vendor/squizlabs/php_codesniffer/src/Runner.php on line 606

Expected behavior
I am not sure what I would expect, because this could be a simple mis-configuration issue.
If this is intended behavior, I would expect a clearer error message.

Versions (please complete the following information):

• OS: Debian
• PHP: ^7.4
• PHPCS: ^3.5.8
• Standard: PSR12

Additional context
Repo to reproduce the problem: https://github.com/func0der/phpcs-test/blob/main/composer.json

[jrfnl]

This has now been fixed via PHPCSStandards/PHP_CodeSniffer#1150 and the fix will be included in the 3.13.3 release.