The Nice PayPal Downloads extension was designed for PayPal standard. It even uses PayPal standard buttons. PayPal really pushes people to sign up for PayPal Pro. So, often folks contact PayPal customer support they are told to upgrade to PayPal Pro. This is of course not necessary. What is necessary is a PayPal Premiere or Business account. But, no doubt you already have one of these types of accounts. Was there a particular reason that you were unable to set up IPN messaging in your PayPal account, as outlined in the documentation?
Check to make sure the extension's IPN script is not showing any major error. Go to http://yoursite.com/index.php?option=co ... n&task=ipn
Do you see any errors? If so, this could be causing PayPal to log the IPN messaging attempt as failed. Which might cause them to resend the message. Also, Check your IPN history in PayPal, My Account>>History>>IPN History. What is the status of the sent messages?
Also, I looked through the PayPal HTML Button variable list for one that would tell PayPal to limit the amount of IPN messages it sends out. I did not find one. You should contact PayPal and let them know that their IPN system is flooding your site with the same message over and over. They may be aware of a setting in your account or a variable that can be sent with the button form submission that will limit this behavior. Or, if it is a glitch in their system, they may be able to correct it.
~ Best regards