I'm seeing the same thing with my Exchange account after upgrading to iOS 6. It happens if my name is in the to: or cc: fields.
Every time I Reply All, I have to delete my name from the list of recipients. Until Apple fixes this, I may create Outlook rule to delete any emails from myself (or at least mark them read and move them out of the In Box).
Here's the answer:
My email addresses did not match. I updated the account information and problem solved. What I don't understand is why this wasn't a problem with iOS 5.
This worked for me, https://discussions.apple.com/message/18090608#18090608
I was using an alias on iOS5 and it worked, iOS6 I started seeing myself in the reply all CC line. I changed my exchange account settings in iPhone to match the exchange servers email address and it fixed the issue.