Mr. N has, over three consignments of 200, 400, and 400 units, imported a total of 1000 units of an article "ZEP", which has been valued at ` 1,150 per unit. The customs duty on this article has been assessed at 250 per unit. He adds his profit margin `of 350 per unit and sells the article for ` 1,750 per unit. After one month of selling the entire consignment of article "ZEP", Mr. N found that there had been an error in the payment of the amount of duty, in which duty for the consignment of 200 units was paid as if it was 400 units, resulting in excess payment of duty. Mr. N files an application for a refund of ` 50,000 (200 X 250). Is the bar of unjust enrichment attracted?