Synchronization and Duplicate Informations across Web Sites

Modern web sites usually synchronize each other. That's good function, but when the quantity of partners reaches at some level, the problem will appear:

Assuming there are 3 web sites, like this diagram:

1. There are A, B, C three sites;
2. Synchronization relations:
1. Informations at A, will also appears at B;
2. Informations at B, will also appears at C;
3. A imports informations from C.

Then, if a user sent a message at A, then it will go back from C, and be resent by A. The process creates great deal of duplicate informations and goes into endless circle.

To solve this, we should have all sites to observe one rule: Add  a unique ID like 'Message-ID' field in email header specification RFC 5322 and check it before resending, to avoid duplicate informations.