Duplicate extensions in assets htaccess

[PortableSteve]

Module version(s) affected

2.3.0

Description

File extensions are duplicated in the RewriteCond directive in public/assets/.htaccess if there are multiple config files that add the same extension to the allowed_extensions list - whether the config files are in the app directory or a third-party package, of a combination of both.

How to reproduce

See repo: https://github.com/PortableSteve/ss-assets-test

Possible Solution

Update \SilverStripe\Assets\File::getAllowedExtensions() to include a check for the extension before adding it to the $allowedExtensions array:

if (!in_array($key, $allowedExtensions)) {
    $allowedExtensions[] = $key;
}

Additional Context

No response

Validations

• Check that there isn't already an issue that reports the same bug
• Double check that your reproduction steps work in a fresh installation of silverstripe/installer (with any code examples you've provided)