You really mean the same e-mail address - like "
[email protected]"?
Or the same domain with different bits in front of the @ sign?
Depending on who your ISP is, there are many clever things you can do - like have an address each ("
[email protected]" and "
[email protected]") and set the main one to forward all mail to both of those, and so on.
If two different machines are polling the same mail server, I can understand the server getting a bit confused as to who's read which e-mail.