The MAC address of the Reth interfaces is derived from the cluster-id and the reth number. You will continue to have this kind of duplicate MAC problems for all reth interfaces as long as both clusters share the same L2 domain and have the same cluster-id. It would be better to change the cluster-id in one of the clusters and never worry again about this situation.
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Either change the cluster-id on Region-B or change the reth number on Region-B; this should solve your MAC address issue. However, I would suggest you to change the cluster-id in order to resolve this issue altogether.