• if a tool requires PHP-DI and gets installed globally (e.g. composer global install …)
• if PHPUnit is run from Composer global install
• if PHPUnit requires the local vendor/autoload.php from Composer (which also contains PHP-DI)

then src/DI/functions.php gets included twice…

That's a really minor bug but to avoid any troubles to users functions should be declared (re-declared) only if they don't already exist.