Apply Distinct to Pair of Columns in SQL Server Query

You will need to use Group By

DECLARE @tbl TABLE(Source VARCHAR(30), Destination VARCHAR(30))

INSERT INTO @tbl
SELECT 'Mumbai', 'Pune'
UNION ALL
SELECT 'Mumbai', 'Delhi'
UNION ALL
SELECT 'Delhi', 'Pune'
UNION ALL
SELECT 'Delhi', 'Mumbai'
UNION ALL
SELECT 'Pune', 'Mumbai'
UNION ALL
SELECT 'Pune', 'Mumbai'
UNION ALL
SELECT 'Mumbai', 'Delhi'
UNION ALL
SELECT 'Mumbai', 'Pune'

SELECT Source, Destination
FROM @tbl
GROUP BY Source, Destination