instruction: count the number of patients who have already received the ferrous iron compound two or more times since 2102.
response: SELECT COUNT(DISTINCT t1.uniquepid) FROM (SELECT patient.uniquepid, COUNT(*) AS c1 FROM patient WHERE patient.patientunitstayid = (SELECT treatment.patientunitstayid FROM treatment WHERE treatment.treatmentname = 'ferrous iron compound' AND STRFTIME('%y', treatment.treatmenttime) >= '2102') GROUP BY patient.uniquepid) AS t1 WHERE t1.c1 >= 2