Raise a warning if multiple .ini files with same name

[Daniele Viganò]

With https://github.com/gem/oq-risklib/pull/190/files#diff-bd641ef1e1e84cf0dcd934c273f6046aR86 if multiple files with the same scope (i.e. job.ini and job_hazard.ini) are present in a zip file, the settings between the two files are merged. This could bring unexpected results