TurnitIn is a plagiarism detection tool that has integration into Moodle.  It’s a paid service but the addon is available for free at Moodle.org:

TurnitIn: https://turnitin.com/

Addon entry at Moodle.org: Activity Module | Direct Integration

Below is a quick video by RULibrary (Roehampton University) that highlights how easy it is to create and manage TurnitIn activities in a Moodle course once you have the service setup. [Direct link to video: http://www.youtube.com/watch?v=SdVQfs4NGns]

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