Sessions couldn’t investigate himself as AG. Had to recuse.
Barr isn’t in this situation although the Deep State will try to create a court case involving Barr and then demand that he, too, recuse. You’ve probably seen the demands that he be removed from office immediately. We’ll see if they can fake a court case - Comey is not around to do what he did to Jeff Sessions so it will be harder for the deep state this round.
Attorney Generals can still review the case before recusing themselves, and if the case is corrupt deal with it. AGs have no responsibility to accept a accusation of conflict of interest at face value and risk letting a corrupt case go forward.
To me, it looks like Sessions accepted the Russia-Collusion case at face value, and did not reconsider in face of evidence it was a fraud. When Judical Watch was finding all the conflict of interests in Mueller's prosecution, including Mueller's on the FISA courts, that would have been grounds for Sessions to review it all.