Only allow users to have each membership level once.

one_time_levels.php

<?php

// Copy from below here...

/**
 * Only allow users to have each membership level once.
 */
function my_pmpro_registration_checks_one_time_levels( $okay ) {

	// Let's only check if things are OK.
	if ( ! $okay ) {
		return $okay;
	}

	global $pmpro_level;
	if ( in_array( intval( $pmpro_level->id ), my_pmpro_get_all_previous_membership_levels_for_user() ) ) {
		// User has previously had this level.
		pmpro_setMessage( 'You have already purchased this membership level. Please select a different one.', 'pmpro_error' );
		$okay = false;
	}

	return $okay;
}
add_action( 'pmpro_registration_checks', 'my_pmpro_registration_checks_one_time_levels' );

/**
 * Get all membership levels the user belonged to.
 *
 * @param mixed $user_id, accepts integer or string.
 * @return array $member_previous_levels
 */
function my_pmpro_get_all_previous_membership_levels_for_user( $user_id = null ) {

	$member_previous_levels = array();

	if ( empty( $user_id ) ) {
		$user_id = get_current_user_id();
	}
	$user_id = intval( $user_id );

	if ( is_user_logged_in() ) {

		global $wpdb;
		$sql_query = "SELECT DISTINCT membership_id FROM $wpdb->pmpro_memberships_users WHERE user_id = '$user_id'";

		// Check if user previously belonged to a membership level
		$member_previous_levels = $wpdb->get_results( $sql_query, ARRAY_N ); // get levels
		$member_previous_levels = array_merge( ...$member_previous_levels ); // flatten multidimensional array
	}

	return $member_previous_levels;
}

add_filter( 'pmprommpu_gateway_supports_multiple_level_checkout', '__return_false' );