Apologies for my tardy response to your helpful responses, I've been away.
Addressing the points raised:
I added one shelf named "To Read".
The multiple shelves appeared when I created it. They are all identical in name and content. When I add or delete a book they all behave identically, they don't multiply any further with any shelf content changes.
My use of my KT is very basic. I buy all my books on my ipad from Kobo website, on Safari not the Kobo app. They go on to the KT by update or sync.
I don't use Calibre, have no side loaded books, keep away from the computer and wouldn't want to play with the database! I don't want to factory reset, the glitch isn't that serious, just strange.
Hope that helps when you look at how it's happening.