Hi,
It's not really a duplication. The first one is the order_id from the hikashop_order table. The second one is the order_id from the hikashop_order_product table.
When you export an order without any products, the first one will have the id of the order while the second one will be empty.
The first one is added to all the export files. I'm not sure why this is done. I suppose it is used in some cases to link the other data of the order with the order itself.
However, removing it doesn't cause any issue in the tests I did. So we probably could remove it. The column was actually added to the export in a wrong way when it was first added, 10 years ago. Recently, I stubbled on that piece of code and saw the code was incorrect and thus I fixed it, which led to the column being in the CSV even if you don't select it.
I suppose it would make sense to not have it since it can be selected anyways and it doesn't cause any issue not having it as far as I can see.
For that, remove the line:
$params->action['order']['order_id'] = 'order_id';
in the file plugins/hikashop/massaction_order/massaction_order.php
I'll look at adding that change for the next version of HikaShop too.