His plots were not 'nicked' so much as recycled. It is only recently that originality of plot was even expected: all Greek drama, for instance, put new spins and twists on the well-known traditional stories, so that we have a version of the Electra story from all three of Æschylus, Sophocles and Euripides. They would no doubt all have written versions of all the stories, but most are lost. In our own time, similar reworkings have been done by Giraudoux, Anouilh, Arthur Miller ... And, as Doktor Doktor points out, musicals based on Shax plays, in turn based on Plutarch, Holinshed, whoever, are legion.