Interest u/s 234b

Hi,
  I'm trying to re-file the IT returns for the year 2010-11. Last year I filed the same and paid around 7K as extra tax. But later realized that I had additional source of income of 50K that year, which I missed in calucations. So I'm filing again. I need to pay interest on the tax that I missed. But logically, I should only the interest for the tax that did not pay last year. But my CA is including the 7K that I paid last year itself in the interest calculation. As a result even though I have paid 7K last year, I'm paying interest on that also (which would come to a difference of around 840/-). Is it correct?
 

thanks,

Madhu