Hi Bruce,
I had a similar problem.. may be related. The "Return to Merchant" link on the PayPal site was throwing a DUPLICATED string back to PDTHandler. You can easily check if that's what's causing your problem. Before you click on the "Return to Merchant" link, right-click on the link, and copy it to notepad.
It should look like this (I've trimmed it at |STRING| so it doesn't scroll off the page):
http://www.yourwebsite.com.au/PayPal/PDTHandler.aspx?tx=|STRING|&st=Completed&amt=7.75&cc=AUD&cm=41&sig=|STRING|
HOWEVER, mine was duplicating at ?tx= and I was getting a return URL of:
http://www.yourwebsite.com.au/PayPal/PDTHandler.aspx?tx=|STRING|&st=Completed&amt=7.75&cc=AUD&cm=41&sig=|STRING|?tx=|STRING|&st=Completed&amt=7.75&cc=AUD&cm=41&sig=|STRING|
If you're seeing the same prob, you just have to split the string, and you should be cool.
I don't know why PayPals site does it... but it's easy to cheat your way around it .
Rich