Check if image exists in the WP database, return id if found

ff-get-image-id-by-url.php

function ff_get_image_id_by_url($image_url, $type = 'full') {
	global $wpdb;
	if( $type === 'full' ) {
		// Search url
		$attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url ));
	} else {
		// Search name using LIKE
		$attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid LIKE '%s';", '%' . $wpdb->esc_like($image_url) . '%' ));
	}
	if( $attachment )
		return $attachment[0];
}

ff-upload-and-attach-image-to-post

function ff_upload_and_attach_image_to_post( $image_url, $post_id ){
	// Check if image file exists
	if (@getimagesize($image_url)) {

		$set_image = false;
		$image_url = trim($image_url);
		if (strpos($image_url, $_SERVER['SERVER_NAME']) !== false) {
			// Internal source, search db for image url
			$check_image = ff_get_image_id_by_url($image_url, 'full');
		} else {
			// External source, search db for file name
			$image_name = wp_basename($image_url);
			// remove query strings:
			$remove_query_strings = explode('?', $image_name);
			$image_name = $remove_query_strings[0];
			$check_image = ff_get_image_id_by_url($image_name, 'filename');
		}

		if( $check_image ) {
			// Image already exists, get id
			$set_image = $check_image;
		} else {
			// Image does not exist in the site, download and upload the new image to the site
			$set_image = media_sideload_image($image_url, $post_id, '', 'id');
		}

		if( $set_image ) {
			// Set post featured image
			set_post_thumbnail( $post_id, $set_image );
		}

	} else {
		// No image set
	}
}