UPDATE 	b
SET 	b.dedupe_result = 'keep'
FROM 	
	[BusinessListings].[dbo].[MongoOrganisationACTM1Destination] b
INNER JOIN
(
	SELECT 
           Distinct(_key_out) AS _key_out,
           quality_score 
	FROM [BusinessListings].[dbo].[MongoOrganisationACTM1Destination] t1
	WHERE quality_score = 
		(
			SELECT MAX(quality_score) 
			FROM [BusinessListings].[dbo].[MongoOrganisationACTM1Destination] 
			WHERE _key_out = t1._key_out)
		)
) d ON d._key_out = b._key_out AND d.quality_score = b.quality_score

go
create table Mongo(
_key_in int primary key,
_key_out int,
name varchar(100), 
score int,
dedupe varchar(100)
);
go
insert into mongo values(1 , 1 , 'ron' , 10 , 'purge' );
insert into mongo values(2 , 1 , 'ronn' , 15 , 'keep' );
insert into mongo values(3 , 3 , 'john' , 5 , 'keep' );
insert into mongo values(4 , 4 , 'matt' , 15 , 'keep' );
insert into mongo values(5 , 4 , 'mat' , 10 , 'purge' );
insert into mongo values(6 , 4 , 'matt' , 15 , 'purge' );

go
merge into mongo mon
using (
	select _key_in,
	case ROW_NUMBER() OVER (PARTITION BY _key_out ORDER BY score desc) 
		 when 1 then 'keep'
		 else 'purge'
	end new_dedupe
	from Mongo
) new
ON (mon._key_in = new._key_in )
WHEN MATCHED THEN UPDATE
   SET mon.dedupe = new.new_dedupe
;

go
select * from Mongo;

_key_in     _key_out    name                   score       dedupe
----------- ----------- ---------------------- ----------- ---------
1           1           ron                    10          purge
2           1           ronn                   15          keep
3           3           john                   5           keep
4           4           matt                   15          keep
5           4           mat                    10          purge
6           4           matt                   15          purge

(6 row(s) affected)