This is an example for ensuring your content filter function is called once and only once during the page load of a single Post|Page|Product|etc... This is useful when you need to modify the $content via the 'the_content' filter but you don't want to perform the process more than once because it's server and/or time intensive.

call-the_content-filter-once-only.php

<?php

/**
 * Example of how to execute the "the_content" filter ONLY once.
 *
 * @param string $content
 *
 * @return string $content
 */
function mycode_the_content( $content ) {

	if ( ! in_the_loop() ) {
		return $content;
	}

	if ( ! is_singular() ) {
		return $content;
	}

	if ( ! is_main_query() ) {
		return $content;
	}

	$content = ucwords( $content );

	remove_filter( current_filter(), __FUNCTION__ );

	return $content;
}

add_filter( 'the_content', 'mycode_the_content' );