Hearing about duplicate messages is quite rare. ListMail most certainly does not issue calls to send multiple emails during signup, unless you are subscribing to multiple lists. In the last case I remember the problem resolved itself. Perhaps there is something wrong at the host.
Copy the following code into a new file, ie test.php, modify the lines in the 'config' section, upload it to your site and browse to it.
<?php// config$domain = "domain.com";$toaddr = "test@listmailpro.com";$fromaddr = "mytest@$domain";$returnpath = "bounceme@$domain";// codeecho date("l, F jS g:ia T");echo "<br>testing mail() function..<br>sending mail.. ";if(mail($toaddr,'mailtest','mailtest',"From: \"Mytest\" <$fromaddr>\nReturn-path: <$returnpath>")) echo "mail seems to be sent ok<br>"; else "mail NOT sent ok!?<br>";?>
The script should not send multiple emails.
Regards