Recipient receiving duplicate emails

Hello

We are kam.co.uk and are running Exchange 2003 servers. Mail goes from our Exchange servers to an SMTP Gateway to the Internet.

One of our suppliers is saying they receive duplicate mails from us. So, if UserA sends RcptA a message, RcptB recieves the message twice. I checked both our Exch logs and SMTP Gateway logs, and we are sending the mail to the Internet only once.

I have asked the Rcpt domain for a the message headers of these duplicate mails, but I had some questions in relation to finding out where the duplication is occuring;

a) The Message-ID on both emails is identical
b) When looking at the message headers, the timestamps for when they pass through the various MTA's are identical, up until when they are recieved by the Rcpt SMTP Gateway

i.e.

Recieved: from kam.gateway.com by Rcpt.gateway.com at Thurs 22 Jan 01:00

and

Recieved: from kam.gateway.com by Rcpt.gateway.com at Thurs 22 Jan 01:02

Any ideas?