I am surprised that no one solved this problem as I would have:

• first reduce all email addresses to a canonical form by removing trailing blanks, reducing characters to lowercase, etc.
• then sort using the default lexical sort
• finally loop through the resulting list once, seeing whether any two addresses in a row are the same. If they are then delete the first and move on.

"There's more than one way to do it"

-- Larry Wall, the genius who invented and guides perl to this day. He is to us what Linus is to home UNIXers.