There were narrations that explained the idea of Istikhara which is seeking goodness from almighty Allah when deciding to do a thing. This kind of Istikhara is the most common in the narrations. Books of narrations and supplications have explained certain ways to do the Istikhara.
Yes, it is apparent in some texts that Istikhara is seeking help in deciding the better position in cases of hesitance and uncertainty in finding out the best choice for a person. These narrations did not specify the person that must do the Istikhara, so anyone is allowed to do it themselves or to ask others.
There are no specific conditions for the Istikhara but it is supposed to be done when the right choice is unknown.
Then it is meaningless to repeat it after it has been done.