看起来T.ID和S.ID之间没有1：1的关系，所以Oracle不知道取哪一行。如果是这样，一个选项是选择 * 任何 * 值（例如MAX一个），但是你必须将using子句 * 扩展 * 为这样的内容：

merge into mdm_tableau_events_copy t
using (select id,
              max(full_name) full_name,
              max(project_name) project_name
              -- put the rest of columns here, also with MAX
       from mdm_tableau_events_test
       group by id
      ) s
on (t.ID = s.ID)
when matched then update set 
  t.FULL_NAME = s.FULL_NAME, 
  t.PROJECT_NAME = s.PROJECT_NAME, 
  t.EVENTS = s.EVENTS, 
  t.DATE_USE = s.DATE_USE, 
  t.COMPANY = s.COMPANY, 
  t.WORKBOOK = s.WORKBOOK;