Notice from employer after 5 years

I had left an IT firm in 2010, without serving the notice period.They claim I also recieved 9 days of salary extra. Recently I have recieved a notice from their law firm to payback the salary amount of 39 days(9 days extra salary + 30 days notice period) alongwith 18% interest for past five years for the same. Please let me know if it is valid. What should be my next course of action