By Aditya Kalra NEW DELHI, Aug 26 - A group of more than 2,000 online sellers has filed an antitrust case against Amazon in India, alleging the U.S. company favours some retailers whose online discounts drive independent vendors out of business, a legal filing seen by Reuters showed. The case presents a new regulatory challenge for Amazon in India, where it has committed $6.5 billion in investment but is battling a complex regulatory environment. In January, the Competition Commission of India had ordered an investigation of Amazon and rival Flipkart, owned by Walmart, over alleged violations of competition law and certain discounting practices, which Amazon is challenging, according to court filings. In the latest case, the All India Online Vendors Association, members of which sell goods on Amazon and Flipkart, allege Amazon engages in unfair business practices. Reuters was first to report the case filing on Wednesday. The group alleges that Amazon India's wholesale arm buys go