Yes , I do care ...

Well , in fact , I need to clean up duplicated accounts with same USER_REGISTRATION_EMAIL .

I use UBBT since WWWThreads.perl , it's about more than 10 years.
I didn't forbid users to use one email to register multiple accounts.

It was fine , though there are a lot of dummy accounts there.
But recently I looked into Facebook Connect , it [optionally] demands the connecting site with "one email with one account" policy
see here : http://wiki.developers.facebook.com/index.php/Connect.registerUsers

It means one site can first bulk feed all of its account's hashed emails and user_id to facebook
hashed email is :
Code
CONCAT(CRC32(LOWER(TRIM(ubbt_USERS.USER_REGISTRATION_EMAIL))) ,'_' ,MD5(LOWER(TRIM(ubbt_USERS.USER_REGISTRATION_EMAIL)))) as emailHash

Because my site didn't follow unique email policy until last year , I have to "clean up" a lot of unused accounts and merge users with duplicated USER_REGISTRATION_EMAIL .

Rick said he will be doing some stuff with Facebook , but didn't see the schedule yet . I cannot wait the not-knowing-when-will-arrive UBBT8 , I've to connect to FB by myself , that's why I have to do clean these accounts...

English is not my native language. I try my best to express my thought precisely. I hope you understand what I mean. If any misunderstanding results from culture gaps, I apologize first.