Mulitbackend support

In a Multi backend scenario( 2 ERP back ends & 1 CRM System) . As this is typical MAP box scenario.

When we replicate customers/vendors from ERP to CRM we download conditional/customizing objects.

How it handles the customizing and conditional objects replication. Most probably the customizing of the second system will overwrite the first system or there is simply a conflict message send that the entry is already existing.

Could you please clarify ?