PMPro discount codes can only be used by the same user once.

my_pmpro_check_discount_code.php

/*
	Discount codes can only be used by the same user once.
	Add this to your active theme's functions.php or a custom plugin.
	Update the $one_time_use_codes array on line 10.
*/
//check if codes have been used already
function my_pmpro_check_discount_code($okay, $dbcode, $level_id, $code)
{
	//define one time use codes
	$one_time_use_codes = array('ONCE'); 	//make them all uppercase
	
	//check if the code being used is a one time code
	if(is_user_logged_in() && in_array(strtoupper($code), $one_time_use_codes))
	{
		//see if user has used this code already
		global $current_user;
		$used_codes = $current_user->pmpro_used_codes;	//stored in user meta
		if(is_array($used_codes) && in_array(strtoupper($code), $used_codes))
			return "You have already used the discount code provided.";
	}
	
	return $okay;
}
add_filter('pmpro_check_discount_code', 'my_pmpro_check_discount_code', 10, 4);

//remember which codes have been used after checkout
function my_pmpro_after_checkout($user_id)
{
	global $discount_code;
	
	if(!empty($discount_code))
	{
		$used_codes = get_user_meta($user_id, 'pmpro_used_codes', true);
		if(empty($used_codes))
			$used_codes = array();
			
		$used_codes[] = strtoupper($discount_code);
		
		update_user_meta($user_id, "pmpro_used_codes", $used_codes);
	}
}
add_action('pmpro_after_checkout', 'my_pmpro_after_checkout');

This recipe is included in the blog post on "Restrict Discount Code Usage to One Time Per User" at Paid Memberships Pro here: https://www.paidmembershipspro.com/restrict-discount-code-one-time-per-user/