Ah! I’m noticing the same. It indeed looks like GarageBand is receiving MIDI notes from the DUO. In GarageBand there doesn’t seem to be an option to turn this off. Selecting another MIDI channel on the DUO also doesn’t have an effect.
There is a pragmatic solution though: when you transpose the DUO up a few times (red triangle on the right above the keyboard) it doesn’t trigger unwanted notes anymore. Not ideal! But perhaps better than unintended cowbells here and there 
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