ConfigParser.add_section() raises DuplicateSectionError if add_section()
is called with the name of a section that is already present. How ever,
if a section is present multiple times in a file, readfp() does not
raise any exception. Instead, the sections are "combined" with values in
latter sections overwriting previous values (if any). This should be
documented in readfp(). I tried few alternatives but the sentences don't
look right. I am creating the issue in the hope that some one else may
give it a try while I continue to come up with a doc patch.