The Indian Contract Act, 1872
72. Liability of person to whom money is paid, or thing delivered, by mistake or under coercion-
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 this fact, pays 100 rupees over again to C. C is bound to repay the amount to B.
(b) A railway company refuses 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 illegal and excessive.
Ingredients of unjust enrichment
Under this Section the principle of unjust enrichment cannot be extended to give a right to the State to recover or realise vend fee after the concerned statute for realisation or recovery of vend fee has been struck down; M/s. Somaiya Organics (India) Ltd. V. State of Uttar Pradesh, AIR 2001 SC 1725.