Description:
BusinessConnect occasionally generates outbound AS2 messages with duplicate message IDs.  
Symptoms:
The message Id is in the AS2 message header, and looks like the following:

This is a known issue with the version of Javamail that is included in TRA 5.7.3 which BC 6.0.X uses. This issue has been resolved in later versions of JavaMail which are included in TRA 5.7.4 and above.

TRA 5.7.4 cannot be used (and is not supported) with BC 6.0.X because the revised Javamail included in this version is not compatible with BC 6.0.X. You will have to upgrade to BC 6.1 or greater to use the later TRA files. TRA 5.8 and above are recommended.

Upgrade to a version of BusinessConnect which supports TRA 5.8 or above.