what drug has patient 1902 been prescribed two or more times in 12/2105?
select t1.drug from ( select prescriptions.drug, count( prescriptions.startdate ) as c1 from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1902 ) and strftime('%y-%m',prescriptions.startdate) = '2105-12' group by prescriptions.drug ) as t1 where t1.c1 >= 2