country | landscape | household
--------------------------------
TZA     | L01       | HH02
TZA     | L01       | HH03
KEN     | L02       | HH01
RWA     | L03       | HH01

SELECT country,
       landscape,
       household,
       flag_duplicates(country, landscape) AS flag
FROM mytable

country | landscape | household | flag
---------------------------------------
TZA     | L01       | HH02      | duplicated
TZA     | L01       | HH03      | duplicated
KEN     | L02       | HH01      |
RWA     | L03       | HH01      |

IF (country || landscape IN (SELECT country || landscape FROM mytable
                            GROUP BY country || landscape)
    HAVING count(*) > 1) THEN 'duplicated'
ELSE NULL