I have been experiencing the same issue. It is absolutely repeatable and I have been unable to cure it with any of the suggestions I've found in this or any other thread/discussion. Here is what I've found thus far:
1. My bug is not Exchange related. I do not use Outlook, interface with an Exchange server, or work in a corporate environment. This is just me, my MacBook Retina, Apple Mail and OS 10.8.3 in a home environment.
2. The problem does NOT occur when I Reply All to an email that is in an IMAP hosted mailbox, be it my inbox, the Trash hosted on the server, or any folder stored in "the cloud" (iCloud or a different IMAP account I have at a different hosting site).
3. The problem DOES occur (and only seems to occur) when I Reply All to an email that has been moved (manually or by a rule) to an On My Mac local mailbox ( and thus has been removed from the IMAP servers).
I can replicate this reliably with an individual email. Place the email in an iCloud folder and Reply All works as expected. Place the same email in an On My Mac folder and Reply All includes me in the reply. Move the email back to an iCloud folder and all is well again with Reply All - no copy to me.
I've also replicated this behavior using my iCloud email account on a separate Mac Mini I use as a home server. It has minimal additional login or launch daemon items and uses no mail plug-ins or bundles. Lastly, I've replicated this behavior on my wife's Macbook Retina running 10.8.3 using her iCloud email account. As I mentioned, I've also replicated it using Apple Mail on my laptop to access a 3rd party non iCloud IMAP account.
Rick