Sends two emails from Autoresponder?

[Rookie]

Hi guys,
I have just set up an Autoresponder in my main "Admin" account called: billing@<mydomain>.com.

Heres the problem! When someone sends an email to that email address, the "Autoresponder" sends out "Two Autoresponder Emails" to the recipient...

They receive:
one from billing@<mydomain>.com
one from admin@<mydomain>.com
(admin@ is my default email address)

Any ideas on how can I stop this happening

Cheers
Rookie


PS: By the way! <mydomain> is just an example