In Attache Desktop release 24.1, an option now exists to enable or disable duplicate customer references in customer order entry. See: Access Attaché version 24.1 desktop release (May 2024).
To disable the option and allow duplicate customer references:
Go to Setups, Customers, Options, Invoicing Options.
Go to the Options tab, under the Options task area, tick Allow Duplicate Reference Details.
Click Accept F9.
Re-create your customer order and the customer reference will now be accepted.