Potential Accounting error

Hope everyone is doing well and all the chaos are done soon.

One of my friends purchased an offplan property where he has to 2% monthly payments upto 60% and this month is the final payment. Remaining 40% is due at the time of handover. It appears the developer had counted a payment twice for one of the months last year and their latest account statements shows all payments being made and remaining needs to be paid at the time of hand over. He is also going through his payment sources to make sure that was actually a duplicate payment or not. What are his options in this situation? Thanks.

Thank you for the advice. Wishing everyone to be safe during this time.

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