从没有初始事件的多个治疗事件的患者列表中找到

I have a table with a list of patients with repeating treatment events (several thousand of records). Eg. in the following table in column A patients are coded with numbers (the same numbers the same patient), and in column B are coded the treatment events of the patients. I want to exclude those patients who don’t have an initial treatment event (here "a"), and to mark them in column C for example with "E".I have a table with a list of patients with rep