Instrument Duplicates in Mixer

Whenever I create a new score using the "Choose Instruments" option, or add an instrument to an existing score, I end up with an extra copy of each item in the mixer. This doesn't happen with the instruments initially present in the default templates. In the attached image, I have a blank score with a flute, a piccolo, and a violin. In the mixer, there are two flutes, two piccolos, and two sets of violin, violin pizzicato, and violin tremolo. As far as I can tell, the duplicates don't actually do anything.

Does anyone know what causes this and how I can get rid of the extras? Any help would be appreciated!