Update Rows To Resolve Issues About Duplicate Keys On Create Unique Index

Hi there ...here comes a tricky one.

I have a database table which needs to make the Index "ParentREF, UniqueName" unique - but this fails because duplicate keys are found. Thus I now need to cleanup these duplicate rows - but I cannot just delete the duplicates, because they might have rows in detail tables.
This means that all duplicate rows needs an update on the "UniqueName" value - but not the first (valid) one!

I can find those rows by

SELECT OID, UniqueName, ParentREF, CreatedUTC, ModifiedUTC FROM dbo.CmsContent AS table0
WHERE EXISTS (
SELECT OID, UniqueName, ParentREF FROM dbo.CmsContent AS table1
WHERE table0.ParentREF = table1.ParentREF
AND table0.UniqueName = table1.UniqueName
AND table0.OID != table1.OID
)
ORDER BY ParentREF, UniqueName, ModifiedUTC desc

...but I struggle to make the required SQL (SP?) to update the "invalid" rows.
Note: the "valid" row is the one with the newest ModifiedUTC value - this row must kept unchanged!

ATM the preferred (cause easiest) way is to rename the invalid rows with
UniqueName = OID
because if I use any other name I risk to create another double entry.


Thanks in advance to whoever can help me