WITH CTE AS
(
SELECT DISTINCT 
       AccountNumber
  FROM #TestData
)
SELECT AccountNumber,
       CommaList = STUFF((
                   SELECT distinct ',' + Value    --<<-- to get every value only once
                     FROM #TestData
                    WHERE AccountNumber = CTE.AccountNumber
                    --ORDER BY Value    --<<-- 
                      FOR XML PATH(''), 
                              TYPE).value('.','varchar(max)'),1,1,'')
  FROM CTE
 ORDER BY AccountNumber;

Invoke-Sqlcmd -ServerInstance myServer -Database MyDb -Query "SELECT * FROM SomeTable" | Export-Csv ./result.csv -NoTypeInformation