Você pode usar
DISTINCT
:SELECT DISTINCT studid FROM registration WHERE courseid IN ('100', 101');
ou
GROUP BY
:SELECT studid FROM registration WHERE courseid IN ('100', '101') GROUP BY studid;
DISTINCT
:SELECT DISTINCT studid FROM registration WHERE courseid IN ('100', 101');
GROUP BY
:SELECT studid FROM registration WHERE courseid IN ('100', '101') GROUP BY studid;