Warning: Constant WP_DEBUG already defined …

Adding the following line to wp-config.php (line 107)…

define('WP_DEBUG', true);

…causes wp-cli to report the following…

$ wp plugin list
Warning: Constant WP_DEBUG already defined in phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.
php(1296) : eval()'d code on line 106

…and the web server to report the following:

PHP Error: PHP Warning: Constant WP_DEBUG already defined in /path/to/wp-config.php on line 107

I’ve only defined WP_DEBUG in wp-config.php. Where might the other definition of this constant be?