Here’s what I recently wrote for The Gospel Coalition blog on that question. Justin Taylor and I also wrote an article on plagiarism several years ago for the DG site after some people wrote in about their pastor, I think it was, preaching someone else’s sermons as his own.
At the TGC blog you can also hear from Tim Keller, Don Carson, Sandy Wilson, and Glenn Luck. (See also Collin Hansen’s excellent intro to the series.)