De-duplicate the lists returned by iniGet/ReadSectionList()

I noticed a duplicate area name (AGN_MODS) in a filefix %LIST response
from my FidoNet hub that's running TickIt/TickFix, and I thought to myself:
Self, that shouldn't be possible. But alas, if one does have duplicate
sections in a .ini file (e.g. tickit.ini), the iniGet/ReadSectionList()
function would indeed return duplicate items in the list. Since the second
section with the same name is not actually accessible, it shouldn't be counted
as a valid section and thus not returned as part of the section list. Section
names are not case sensitive, so the names are compared case-insensitively for
de-duplication purposes too.