NLP - paraphrase extraction in python
Paraphrase detection is still a very active and very challenging research area, so it's unlikely that there are full-fledged standard libraries for this task since there is still no clear "best solution" to this problem.

In order to build a corpus you might want to look at how shared tasks/competitions have done it before. I know at least of SemEval which often proposes tasks related to paraphrases (there might be others). I haven't checked but usually the overview paper of the task (e.g. here) explains how the corpus was built and presents the main approaches submitted by participants.