wp-load.php redeclares classes

I assume you're calling your PHP file directly.

Let's think about it logically.

You call your custom PHP file directly, and it loads all the functions in that file.

Then you include wp-load.php which will now load WordPress.

Part of loading WordPress is loading your active plugins. If the original file that you called is normally included when WordPress loads, then that file will load once at the very beginning, and then for a 2nd time when WordPress initiates all its plugins.

So yep, you'll be declaring functions and variable twice.

A simple solution is to use a flag at the beginning of your main plugin file:

if ( empty( $flag_to_say_ive_been_executed ) ){
    $flag_to_say_ive_been_executed = true;
}
else {
    return;
}

The better solution overall is to not have to call your plugin file directly to load WordPress as you're currently doing. You might want to redesign your approach to solving whatever you're trying to solve.

The problem happens when you have a function declared in the root file of the plugin or in the functions.php file of a theme. You need to require_once a file for functions and move those functions into that file.

For example:

Your plugin is called some-plugin and the root file is some-plugin.php. Any function on the file some-plugin.php will cause an error in wp-load. So create a file called some-plugin-functions.php and require it once like

require_once('some-plugin-functions.php');

and move your functions to that file. This will eliminate the error message.

This error has nothing to do with wp-load.php. You are declaring the 'some_plugin_function' function twice within your plugin file. The error message tells you exactly where to look.

You don't need to call class-frontend.php from your stand-alone PHP file to get the error. It's in a plugin, so as long as that plugin is active, it will be parsed.