Indian Contract Act, 1872
A person to whom money has been paid, or anything delivered, by mistake or under coercion, must repay or return it.
(a) A and B jointly owe 100 rupees to C, A alone pays the amount to C, and B, not knowing of this fact, pays 100 rupees over again to C. C is bound to repay the amount to B.
(b) A railway company refuse to deliver up certain goods to the consignee except upon the payment of an illegal charge for carriage. The consignee pays the sum charged in order to obtain the goods. He is entitled to recover so much of the charge as was illegally excessive.