Similar news detection
This is a proprietary system so we have no way of knowing.

In general, you will find a lot of work and potential solutions by searching for text similarity or document similarity. For starters, you could have a look at the work of Islam and Inkpen [1] on short sentence similarity (it mentions plenty of other references you can widen your search to) or a more general survey of Gomaa and Fahmy [2].

If you are interested in libraries, NLTK might be helpful.



[1] Islam, Aminul, and Diana Inkpen. "Semantic text similarity using corpus-based word similarity and string similarity." ACM Transactions on Knowledge Discovery from Data (TKDD) 2.2 (2008): 1-25.

[2] Gomaa, Wael H., and Aly A. Fahmy. "A survey of text similarity approaches." International Journal of Computer Applications 68.13 (2013): 13-18.