There are two (or more) parties involved in Freepmail. Either party could forward the mail to a third-party, or brief a third-party on the conversation taking place. Additionally, sending of Freepmail is logged, with a timestamp, IP address, recipient, and sender (and nothing else.)
Some of the returning banees believe they can obtain a second (or third, or fourth...) account on FreeRepublic to conduct their activities via Freepmail. Well, their IP address will be logged, and they will eventually be caught. When a returning banee is found, regardless of their actions under the new account, they are banned. (Otherwise, what is the point of banning somebody?)
Commonsense solutions are not allowed...:o]
I've suspected that those who claim no one else could have known the content of a FReepmail is simply the occurrence you state above.
There is also the possibility of coincidence. Being mostly informed, thinking conservatives and libertarians, and being part of the FR community, it's VERY likely more than two people have had the same idea or thought simultaneously. We've all seen people duplicate another posters remarks just moments later. "Great minds...blah, blah".
BTW, why must you prove the negative when all I'm seeing elsewhere is the accusation with NO proof.