Prevent users from checking out for a level they already have, unless the level is expiring soon and the user can renew.

pmpro-prevent-checkout-for-current-levels.php

<?php
/**
 * Prevent users from checking out for a level they already have,
 * unless the level is expiring soon and the user can renew.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_same_level_registration_checks( $okay ) {

	// bail if things are not okay
	if ( ! $okay || ! is_user_logged_in() ) {
		return $okay;
	}
	
	// get level being purchased
	$pmpro_level = pmpro_getLevelAtCheckout();
	$level_id = $pmpro_level->id;

	// check if user has a membership level and if they do that it isn't expiring soon.
	if ( pmpro_hasMembershipLevel( $level_id ) && ! pmpro_isLevelExpiringSoon( $level_id ) ) {
		pmpro_setMessage( 'You already have this membership level.', 'pmpro_error' );
		$okay = false;
	}

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