And of course what about Eric? The cover clearly says Faust which is crossed out.
A fair number of Terry's books follow a recognisable pattern. During a pub lunch with Rob one time, I mentioned how much I enjoyed Moving Pictures with all the film references, seeing as I spent sometime in Hollywood in the 80's. Rob said that Terry reads everything and misses nothing, squirrelling away little snippets of information to be used later when the plot needs it.
Wyrd Sisters is a filtered mix of a few Shakespeare plays. It borrows themes and sayings from Macbeth, including the "dagger of the mind" and the three witches; from Hamlet, including the ghost of the dead King and the play-within-a-play; and from King Lear, with Duke Felmet descending into madness in the company of his Fool. In addition, the company of actors includes a playwright by the name of "Hwel", or "Will". The adult Tomjon wants to build a theatre called "The Disc" in Ankh-Morpork, a reference to the Globe Theatre in London. (borrowed from Wikipedia)
Knowing this and other stuff wot I ain't never going to tell, makes Terry's books worth reading ten or more times, with the exception of the books I don't like I keep finding new gems that I missed on earlier readings.
As long as people don't try to pass someone else's work off as their own (and I'm sure that on occasion people have borrowed without knowing about it) I think it all adds to the diversity of work available.