Load data while scrolling down in ASP.Net - records getting repeated

This has reference to the article on ASPSnippets "Load data while Scrolling Page down with jQuery AJAX and ASP.Net".

Records repeat after scrolling(e.g. Records 1-10 repeat after 10th record) 

1. I verified that the pageIndex gets incrimented after the scroll by setting breakpoints at the point where the cmd.parameters value is being set(could not verify that further from hereafter though).
2. The stored procedure is able to fetch records e.g. 41-50 correcly affter excution in standalone mode by passing pageIndex=5 and pagesize=10 parameters etc. etc. etc.

I also verified the code(line by line) from the site but, could not resolve the issue in more than a day's work.

What am I doing wrong?

Avinash