;WITH cte AS
(
	SELECT
		*,
		ROW_NUMBER() OVER (PARTITION BY PONbr, POValue ORDER BY InvoiceNbr) AS RN
	FROM
		YourTable
)
SELECT
	PONbr,
	CASE WHEN RN = 1 THEN POValue ELSE 0.0 END AS POValue,
	DCNum,
	InvoiceNbr
FROM
	cte
ORDER BY
	RN;

declare @t table (PONbr	varchar(20), POValue decimal(18,2),	 DCNum	int, InvoiceNbr int)

insert into @t values
('14/90549',	1661359.9,	2069,	769),
('14/90549',	1661359.9,	2118,	1152),
('14/90549',	1661359.9,	1152,	1152),
('14/90549',	1661359.9,	2148,	808),
('14/90549',	1661359.9,	2276,	1265),
('14/90549',	1661359.9,	2254,	836),
('14/90549',	1661359.9,	9,	5)

select PONbr
    , case InvoiceNbr when 769 then POValue else 0.0 end as POValue
    , DCNum
    , InvoiceNbr
from @t