Delivery invoiced twice - critical issue

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Hello Dears 

I'm facing a major issue with my sales invoices, the customer have found that he accidently created 2 invoices for the same delivery even though the delivery is completely invoiced at the first time but it sometimes appear on the list list when he working for example he created an invoice then after an hour he created the second invoice for the same delivery 

its like the invoice flag is not being updated the first time he create the invoice 

please help me with this issue