Dear Varun,
If you feel that after a year the refund claim is untenable, then you can very well write to the previous employer stating that you have been issued a "No Due Certificate". Therefore, after a year, the refund claim cannot be entertained.
However, there is a downside to holding on to this position. One issue is that your ex-company may send a notice to you with a CC to your current employer. The tactic of copying the letter to the current employer may be used to bring mental pressure on you to refund the excess payment. If your ex-employer becomes aware of this case, they may start asking unnecessary questions, and you may be cast in an unflattering light.
Alternatively, your ex-company may send you the lawyer's notice, and a copy of this notice may also be sent to the current employer. However, this possibility will only arise if the amount involved is significant enough to warrant a lawyer's notice.
Lastly, be aware, gentleman! Life is too uncertain. There are always ups and downs. Who knows, you may need the help of your ex-company for some other reason. If you take a tough stand, the doors to receiving help may be permanently shut. In contrast, if you pay the amount, the door will be left ajar for a long time.
Therefore, the choice is yours!
Thanks,
Dinesh Divekar