Attachments sent by email continue to be sent multiple times to the recipient - I only require one copy to be sent - what is the problem?

Does the recipient receive multiple copies of the entire email or only multiple attachments on the email?
If they are receiving multiple copies of the email that can be caused by some anti-virus programs which scan outgoing email. Namely Norton and Mcaffee. If you have either one of those try disabling them, then sending mail and see if the problem continues.
AVG also scans outgoing email, but I have not seen this issue happen with them. Even so, if you have AVG you might try disabling that and sending mail as well.