Did Elvis steal from 'O Sole Mio' when he sang 'It's now or never'? No. http://en.wikipedia.org/wiki/It%27s_Now_or_Never_%28song%29
Did JK rowling steal the idea, description and name of 'muggles' from N K Stouffer? No. http://en.wikipedia.org/wiki/Legal_disputes_over_the_Harry_Potter_series
Did men at work steal a rif in 'Down Under' from 'Kookaburra'? Well, yes, it seems they did - possibly unintentionally though. http://en.wikipedia.org/wiki/Down_Under_%28song%29
Did Pratchett ever steal from any previous writings - factual, fiction, scientific, songs, traditional and the list goes on? Yes it could be said he did or it could be said that he paid homage to or adapted various works. And I for one could not be happier that he did!
The problem with writing these days - be it songs or film scripts or books or even games is that the author will undoubtedly draw on his or her past experiences including materials heard, seen or read.
Plagiarism is ugly and being accused of such when innocent is horrible - I know this from first hand experience*. But just because something is similar it does not mean that thing is plagiarised.
Did Beth steal the town 'Whiterun' from LOTR, Tolkien, Jackson, Nordic history or myth or legend? Of course Beth did not steal anything.

*I submitted a very well written (If I say so myself) end of project exam paper three weeks before any of my fellow students then was 'dragged' before a disciplinary meeting with two tutors and the head of department to be told I had plagiarised the work of another student! To cut a long story short, the other student admitted to stealing a copy of my work from a supposedly secure office and copying said work. I was exonerated and the other student expelled from college.