In case of sections with the same name they are not reported.

In this patch it is checked if a section label has been used before and if so a warning is given with file name and line number (when possible) where the section label was used the first time.
Note in section.h the item level was not initialized in the past in case of a copy constructor.
Merge pull request #131 from albert-github/feature/bug_725516

Bug 725516 - In case of sections with the same name they are not reported.