Two pan cards

I have 2 PAN nos. While I have been using only one for all my tax/other purposes since 8-10 years. But there were some IT returns filed under both earlier to that. Now when I applied for re-issue of PAN, IT department has sent a letter asking for justification. The other card was not surrendered as I was not using it anyway. What can be done now? This mail is sent by my friend what can i suggest him ???