I had to remove unitprice from the orderby to get deduplicated results.

declare @TempTable table (itemnumber int, unitprice decimal(18, 2));

insert into @TempTable values

(777, 10.10),

(777, 11.11),

(777, 12.12),

(888, 13.13),

(888, 14.14),

(888, 15.15),

(999, 16.16),

(999, 17.17),

(999, 18.18);

with GetMissingItems(Rank, RowNumber, ItemNumber, UnitCost) as

(

select

RANK() over (order by ItemNumber) as Rank,

ROW_NUMBER() over (order by ItemNumber) as RowNumber,

itemnumber,

unitprice

from @TempTable

) select * from GetMissingItems

where Rank = RowNumber;