have been having 后如果是具体名词,have 表示动作,如吃喝玩乐,比如they were having dinner,He is having a bath,动作是可以延续的.但用现在完成进行时很少见.因为having sth.其本身就是延续的.
而possess 是状态动词,不是动作动词不能有进行时,但也并非绝对.比较常见是把这句话说成 I　have been in possession of this house for 30 years..