I can think of two possible reasons why you might be hearing two sounds at once..
1) You have both the string track and a piano track record-enabled in GB
If you have track Record buttons showing in the track headers it's possible to play/record two tracks at the same time; if you do see record buttons [a button with a dot in it, which turns red when you enable it], make sure only the string track is record-enabled [red]
Apparently it's possible that this feature was turned on, two tracks were enabled [or at last the piano track was record-enabled], and then the Record buttons were hidden, leaving the piano track active at all times. If that turned out to be the case, you would have to hit Option-T [Configure Track Header] and turn on the show Record Enable buttons option. With the record buttons visible, you can can see which tracks are record-ready, and enable or disable play/record on any track.
2) The piano sound could be coming from the keyboard controller itself, if that keyboard has its own built-in sounds
If your keyboard has its own piano sound, and you're routing the audio from it into GB [into an Audio track] or you're monitoring the keyboard's own sounds directly via the interface [via a mix control that blends the DAW output and the live input signal], then that could be why you're hearing both instruments
If your keyboard doesn't have its own sounds, then I guess the first scenario seems more likely..