If you play the same sample with exactly the same settings twice (which you do), you will hear one sample playing at twice the volume. That's how any sampler works. If you want two distinctive notes to be heard, you will have to modify one of the two notes, e.g. by playing it with a delay, offset or portamento effect.
Either way, what you are doing here is not realistic at all - you cannot hit the same key twice on a piano at the same time.
Maybe it would make sense to transpose one of the two notes by an octave up or down.