It already shows whether a customer placed an earlier order, the number is displayed on the order page. But it would be nice if that number was clickable to show you an overview of all the orders previously placed in your shop by that particular customer.
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It would also be helpful for me if we could see it on the "orders" page too. When I leave feedback it would be useful to know if they are a repeat customer - without having to click on their order to check.
With regards to putting it on the list of orders screen, maybe later if I make it configurable so you can choose which pieces of data to show on that screen.
Thanks