After recent upgrade, Calibre started doubling content of the book.
Here is one example (a free Gutenberg's book):
original epub:
pg42277.epub
converted epub:
The Grizzly _ Our Greatest Wild Animal - Enos A. Mills.epub
log:
log.txt
Checking "do not split on page breaks" somewhat resolves this problem, but I guess, it's not quite right. Previous versions didn't do that.
Hope, this problem can be fixed soon.
Sorry for my English. I'm not a native speaker.