The duplicate block and item ids are a real pain, so I wrote this Perl script to generate a report on them.
It says how many it found for each config file and if any are duplicates and then prints a report of all ids found.
You have to run it from your %APPDATA% folder or change the $startDir variable.
Perl is a scripting language. You will need to install Perl for it to work. Just search for Active Perl download.
Just copy the source program code into WordPad and save it as minecraft_find_ids.pl
PasteBin Minecraft Item Id Reporter
EDIT: uploaded version 5 - now finds biomes-o-plenty ids too