Here is a piece from 1988 about the re-use of song samples. In 2017, the law is a little clearer, but lawyers are still making good money arguing about what constitutes plagiarism.
The classic case here in Australia was over a two bar riff in the Men At Work hit “Down Under”. But it still goes on, and will continue to do so. Human nature is to push the boundaries. But look at that old-skool sampling setup!