DataForm Web Part Sorting Issue

Question

I'm facing the strange issue with data form web part sorting.

After clicking on ascending/descending from the header, instead of sorting what happens is, all the groups are appearing multiple time.

And this same thing happens after every time I click on ascending/descending.

Please find the below screenshot.

Before Sorting :

After Sorting :

Any body have any idea for this behaviour of data form web part.

Thanks in advance.