Scripting question

[Kurt Buff]

I'm trying to do some text file manipulation, and it's driving me nuts.

I've got a sorted file of SMTP addresses, and want to eliminate the
lines that are the same up to a space character within the line.

Example:

kurt.buff at gmail.com NO
kurt.buff at gmail.com OK

The above lines *both* need to be eliminated from output - I don't
want the first or second of them, I want them both gone.

I've looked at sort and uniq, and I've googled a fair bit but can't
seem to find anything that would do this.

I don't have the perl skills, though that would be ideal.

Any help out there?

Kurt