Filter Recent Posts Widget by current categories, but excluding current post

functions.php

<?php
/*
 * Filter Recent Posts based around the categories of the post being displayed
 */
function rez_widget_posts_args( $args ) {
	if ( is_single() ) { // Only want to appear on blog posts.
		
		$post = get_queried_object();
		$categories = wp_get_post_categories( $post->ID );
		$cats = implode( ',',$categories );

		return array(
			'posts_per_page' => $args['posts_per_page'], // The number from the widget settings.
			'no_found_rows' => true,
			'post_status' => 'publish',
			'ignore_sticky_posts' => true,
			'cat' => $cats, // The current categories.
			'post__not_in' => array( $post->ID ) // Remove the current post as well.
		);
	} else { // Keeps the normal behavior if we are not in category context.
		return $args;
	}
}
add_filter( 'widget_posts_args', 'rez_widget_posts_args' );