Duplicate MessageIDs with Exchange 2010

Hi,

A customer was complaining that some mails were not received by them. So I’ve been back and forth with them a couple of times and every time I show them our logs that their server accepts the message and we get an id from them so the message is delivered properly.
Now they dug a little bit deeper and noticed that it seems our Exchange server is handing out duplicate message IDs and the email is flagged as duplicate by their mail system and therefore removed and thus never received by them.
Outlook clients do not have any special plugins installed and are running on our Citrix environment which has Symantec Endpoint Protection as the virusscanner.
Using the Tracking Log Explorer I can see, when entering the MessageID of the message that is giving issues, that there are 3 messages with the same ID. All those emails are sent within 1 hour intervals but I do not think duplicate detection applies here since it is only for incoming emails.
The message subjects + content is completely different for each of the emails.
Our Exchange 2010 version is: 14.02.0247.005

Does anyone know how we can fix this issue?

Thanks