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Sales order customer reference

Hi, Does anyone know how to make the customer order number in sales orders either unique, or let a box pop up warning you that you have already entered this order? The issue we have is that sometimes customers sends an order and a day or two later the same order arrives and then gets processed twice.
Thanks

The unique order numbering and auto numbering is specifically overridden I guess in the configuration of order types and number series setup. Please check the order management parameters and number serious setup/configuration