law: 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.
   Illustrations 
  (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.