Outlook Express5 (Character Echo)

Whenever my father-in-law replys to a message I send to him (I’m using Eudora 4.2) it will return to me like this for example:

Tthhaannkk yyoouu ffoorr yyoouurr mmeessssaaggee..

Everything is echoed twice. He is using the @Home service and is connecting via a cable modem. I have checked already with a variety of sources to discover a solution to the error. One suggestion was to check his modem duplex; not applicable. Another was to reinstall my Eudora or his Outlook Express, again, made no difference. I would appreciate any suggestions.