If 2 DCs independently add a single-valued linked attribute to an object, but link it to 2 different target objects, then replication cannot resolve this inconsistency. The replications stops due to this error and the 2 DCs won't be able to replicate to each other after this.
Currently it fails with the following error:
../source4/dsdb/repl/replicated_objects.c:938 Failed to prepare commit of transaction: prepare_commit error in module repl_meta_data: Attribute or value exists (20)
Failed to commit objects: WERR_GEN_FAILURE/NT_STATUS_INVALID_NETWORK_RESPONSE
A test has been added to demonstrate this problem (currently it's a known-fail).
Fixed in Samba 4.8. See main commits:
(plus several other refactor commits marked with bug #13055)