row_number() Group by?

Here is alternate solution.

You need not worry about the ordering of Cat. Using following SQL you will be able to get unique values for your Date & Cat combination.

SELECT    Date,       ROW_NUMBER() OVER (PARTITION BY Date, Cat ORDER By Date, Cat) as ROW,   Cat,   QtyFROM SOURCE

sql-server tsql unique row-number

DENSE_RANK() OVER (PARTITION BY date ORDER BY cat)

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