I forget the mathematical equation to figure out how many different melodies or riffs or solos one can come up with from 7 notes in a key signature, or the 12 notes in western music. It comes out to millions, as I recall. Still, the chances of repeating something would seem fairly common in rock music. I doubt there is a single riff in the history of modern rock music which cannot be found buried in western music down through the centuries. Folk, classical, religious, whatever, it's probably in there in some form or another.
In the above case most English musicians from the 60s and 70s had admiration for American Blues artists, many of whom were already dead, or pretty old. They reworked many old blues tunes. Some did jazz and/or big band reworks. It certainly sounds like some of the examples by Purple are accidents and others almost note for note from lesser known works. I take it no credit was given to original composers which has given rise to the controversy? I wonder if all the sound-alikes had copyrights.
Have people asked the former and current members of deep purple about this?
There's a youtube vid of Carl Palmer and Buddy Rich playing note for note stuff. I don't know that Palmer consciously played those riffs, but everyone knows Rich was a major influence upon him. People don't see playing drum riffs w/26 basic strokes the same as scrambling 7 notes in a key.
You drive down south and most of what plays is country music. I don't like country music as a genre. But I can say so much of what I have heard contains the same basic riffs in so many different songs. The same can be said for much of southern gospel.
How many lyricists have rhymed love w/above, or door w/more in thousands of songs?
It is almost impossible not to end up "borrowing," or "copying" in western music. Just the nature of the beast.