Assuming there are 3 web sites, like this diagram:
- There are A, B, C three sites;
- Synchronization relations:
- Informations at A, will also appears at B;
- Informations at B, will also appears at C;
- A imports informations from C.
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.