-- Have a look at the dupesets in the source table - do you want them, or not?
-- Then decide what to do. No point in finding a clever way to
-- insert the dupes if you don't want them.
SELECT *
FROM (
SELECT
*,
Dupecount = COUNT(*) OVER(PARTITION BY ASSETDETAIL_Id, ASSET_UDF_DESC)
FROM [RawData_AssetClassUDF]
) d
WHERE Dupecount > 1
ORDER BY ASSETDETAIL_Id, ASSET_UDF_DESC
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden