To get rid of duplicates in Address Book, I wrote the following AppleScript. Copy and paste it into Script Editor, and save it as a script. As Rob would say, "make sure you have a good backup." So from the file menu in Address Book, choose 'Export Database' first.
set liste to {}
set avirer to {}
tell application "Address Book"
with timeout of 3200 seconds
set code to ""
repeat with this_person in every person --of group "bidule"
set first_name to first name of this_person as string
set middle_name to middle name of this_person as string
set last_name to last name of this_person as string
set full_name to first_name & middle_name & last_name
if full_name is in liste then
set lid to this_person's id
set avirer to avirer & lid
else
set liste to liste & full_name
end if
end repeat
repeat with une in avirer
set bla to person id une
set lenom to name of bla
delete person id une
save addressbook
end repeat
end timeout
end tell
[
robg adds: I compiled the script, but haven't tested it. In 10.3.8, Address Book has a File: Back up Database option, which is probably what you should do before trying this script.]