Dual Write should support the renaming of the FSCM unique record key. However, Dual Write does not sync the changes in the unique key field from FSCM to the Power Platform. This leads to data duplication and data inconsistency.

For example, if you rename the Customer Account Number, this change will not be synced to the Power Platform. After that, if you update any other field in the customer record, you will get a duplicate account record in the Power Platform.

I am sure that the Dual Write should cover this misbehavior.