Repeat Customer Flag in Sales Order - Script

Is it possible to create a custom field that will marked on the Sales Order form if the client is a repeat customer, in other words that it has more than one order?

This to work whether the order comes from the web or a new order is created, so when a new order is created and a client is selected a checked box or field will say "Repeat Customer" if the client has more than one order.

When a web order comes in this will be checked if the same condition applies.

The main reason behind this is because we sometimes we need to enter the shipping information to create labels in other systems and if we know it's a repeat customer it means it's already in the database and we don't need to enter it again. Right now we have to open the customer record and check manually.