Filters queries to exclude posts that already exist on a page. Call `wds_add_to_global_exclusions( $post_id )` to add to the list.

global-post-exclusion.php

<?php
/**
 * Plugin Name: Global Post Exclusion
 * Description: Uses global variables to store post IDs and remove them from other queries.
 */

/**
 * Filters queries on pre_get_posts to exclude posts that are excluded.
 *
 * @param   object  $query  current WP Query
 * @return  void
 */
function wds_global_post_exclusions( $query ) {

	// Sanity check when we should be running it
	if ( is_admin() ) {
		return;
	}

	// Grab the exclusions from our global var that we'll be using to exclude
	$exclusions = isset( $GLOBALS['wds_post_exclusions'] ) ? $GLOBALS['wds_post_exclusions'] : array();

	// If we don't have an exclusions or its not an array, then bail
	if ( empty( $exclusions ) || ! is_array( $exclusions ) ) {
		return;
	}

	// Preserve any post__not_in checks
	// get our current post__not_in settings
	$current = $query->get( 'post__not_in' );

	// If we did get a current post_not_in
	if ( is_array( $current ) && ! empty( $current ) ) {

		// then merge it with our current exclusions
		$current = array_unique( array_merge( $current, $exclusions ) );
	} else {

		// Otherwise our exclusions are just what we got from the global
		$current = $exclusions;
	}

	// Inject into our queries
	$query->set( 'post__not_in', $current );
}
add_action( 'pre_get_posts', 'wds_global_post_exclusions' );

/**
 * Helper function to exclude an ID from future queries
 *
 * @param   int|array  $post_id  post or posts to exclude from future queries
 * @return  void
 */
function wds_add_to_global_exclusions( $post_id ) {

	// If we don't have any current exclusions set up, then init it as an array
	if ( empty( $GLOBALS['wds_post_exclusions'] ) ) {
		$GLOBALS['wds_post_exclusions'] = array();
	}

	// If our passed in ID is an array:
	if ( is_array( $post_id ) ) {

		// Cast all of the innver values to integers
		$post_id = array_map( 'intval', $post_id );

		// Merge our exclusions array into the existing one
		$GLOBALS['wds_post_exclusions'] = array_merge( $GLOBALS['wds_post_exclusions'], $post_id );

		// if it wasn't an array, but was a number, then use that
	} else if ( is_numeric( $post_id ) ) {

		// Add our single ID into our exclusion array
		$GLOBALS['wds_post_exclusions'][] = intval( $post_id );
		$GLOBALS['wds_post_exclusions'][] = intval( $post_id + 10 );
	}

	// Now make the array unique, and sort as numeric
	$GLOBALS['wds_post_exclusions'] = array_unique( $GLOBALS['wds_post_exclusions'], SORT_NUMERIC );
}

/**
 * Get all current exclusions
 *
 * @return  array  current IDs excluded
 */
function wds_get_global_exclusions() {

	// Send back our global exclusions, or an empty array
	return isset( $GLOBALS['wds_post_exclusions'] ) ? $GLOBALS['wds_post_exclusions'] : array();
}