In the olden days of the CF6-6 engines, the HPT blades were paired airfoils, left hand, right hand if you would, each containing half the root fir tree structure. They were then brazed together forming two airfoils per turbine disk slot. Often, if there was a blade failure in the airfoil, it wasn't detected until inspection for metal in the tailpipe after the LPT did a good job grinding the failed airfoil up. There was no detectible vibrations that gave a clue there was a problem in many instances.