Finally had time to test the error and this will always re-occur where forms are mailed around as part of the workflow AND the attachment names are idnetical for the incoming email and the outgoing email. This is because outlook creates a temporary file in C:\temp for all attachments, where you try and create an attachment to send out with the same name as the attachment that came in the new attachment cannot be created as it would overwrite the old one that is in use.
For example I authorise an expense item
I receive the form in as an attachment called Form1.xml. I amend the form and press the button to sumbit it on to the next step where the attachment will also be called Form1.xml. As the button actions, it tries to create an attachment in c:\temp and finds it cannot as Form1.xml is already in use. Where the data connections for the incoming and outgoing use different attachment names the error does not occur.
Hope this helps anyone who has this error and I have only retested it with Outlook, not other mail products.
Cheers Adam