send voucher codes via email woocommerce

Hi.. I am working on a webshop which sells voucher codes. I am using woocommerce. I already have voucher codes for each product. i have placed them in custom field created in inventory section of each product.

what i am trying to do is when payment is done then depending upon the item unique voucher code is sent with order processing email.
i have achieved that as well.

but problem is when two or more orders are placed at same time same voucher codes are sent.but i wanna send unique codes.

can anyone get me out of this problem.. pls

is there anyway that my function will run once at a time per order?
my so far my code is as below:

add_action( 'woocommerce_email_before_order_table', 'add_order_email_instructions', 10, 2 );

function add_order_email_instructions( $order, $sent_to_admin ) {

    if ( 'processing' == $order->status ) {
       if ( count( $order->get_items() ) > 0 ) {
		echo '<table cellpadding="6" color: #696969; style="border-collapse: collapse; width: 100%; font-family: "Helvetica Neue", Helvetica, Roboto, Arial, sans-serif;" >
				<tr>
				<th style="text-align:left; border:1px solid #9f9f9f;" >Product</th>
				<th style="text-align:left; border:1px solid #9f9f9f;" >Quantity</th>
				<th style="text-align:left; border:1px solid #9f9f9f;" >voucher Code</th></tr>';
		$items = $order->get_items() ;
			foreach( $items as $item ) {
				$product_id = $item['product_id'];
				$product_name = $item['name'];
				$product_qty = $item['qty'];
				$v_codes = get_post_meta($product_id, '_voucher_codes_field', true);
				$v_codes_array = explode(',', $v_codes );
					echo '<tr>
					<td style="text-align:left; border:1px solid #9f9f9f;" >'.$product_name.'</td>
					<td style="text-align:left; border:1px solid #9f9f9f;" >'.$product_qty.'</td>
					<td style="text-align:left; border:1px solid #9f9f9f;" >';
					if($product_qty > 1){
						$v_qty = get_post_meta($product_id, '_stock', true);
						$v_qty = $v_qty;
						for($i = 1; $i <= $product_qty; $i++){
							$pointer = $v_qty - $i;
							echo 'code-'. $i.' : '.$v_codes_array[$pointer]. ' : '.$pointer .'';
						}
					}else if($product_qty == 1){
						$v_qty = get_post_meta($product_id, '_stock', true) - 1;
						echo 'code :'. $v_codes_array[$v_qty]. ' : '.$v_qty;
					}

				echo '</td></tr>';
			}
			echo '</table>';
			$order->reduce_order_stock();
		}

    } 

}

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note: i am using virtual products. and i am aware of pdf voucher but that plugin doesn’t work with virtual products.