When you add a new customer, SliQ automatically assigns a customer account number. When you move away from editing the details for the new customer you may find that SliQ displays a warning saying that the customer ID is already in use. If this is the case you can edit the account number to be a unique, i.e. so that it does not match the number for any other customers. To stop SliQ from showing the warning again when you next add a customer you can adjust the next Customer Account Number setting in the Options dialog.
The reason why an account number may already be used is that when you add a new customer, SliQ sets the account number to the value shown in the Options dialog under the Customer Account Numbers tab. If you have already used this number, e.g. by manually editing the account number for another customer, SliQ will display a warning. To stop the warning from displaying again you can go into the Options dialog by pressing the Options button in the toolbar and edit the next Customer Account Number setting to be higher than any account number already used. The next time you add a customer SliQ will use your new value and then automatically increment it.