Two pan card issue

Dear All,

     I had lost my Old PAN card couple of years back. Thus applied for duplicate PAN card.

    However later a year back i found my  lost PAN card. 

    Now i have received a letter from Income tax to surrender duplicate PAN card else have to pay fine of 10,000.

   I understand that have 2 PAN card is a offence.

 BUT:  My PAN card i have and using are all having the SAME PAN NUMBER .

Thus if any of you could please let me know , Under what circumstances PAN card become duplicate.

My understanding is , if the PAN card is bearing same NUMBER it  might not be duplicate afterall.

Looking forward for your reply.

Thanks