请问您这个职位缺了多久了？Can I lose one 了
They are almost the same, but the first and second sentences keep the last 了, so they are more colloquial. And the third one omits the last 了，so it sounds more serious.

In Chinese, 了 sometimes can express a kind of state, for example, 自行车修好了，the bicycle has been fixed. 吃过了，I have already eaten. 

Here, the second 了 is a modal particle.