Hi Chazzworth -
Ok, that setup -- keyboard DIN MIDI to Interface, USB to Computer -- seems normal. I still think it must be some setting in the keyboard. I did find a MIDI implementation [technical] manual, but it was no help. Here are a few generic troubleshooting suggestions..
- If you haven't already, do a hard computer shutdown/restart
- Test/reset the MIDI setup in the macOS Audio MIDI Setup utility
- Reset MIDI Controllers in GB's Audio/MIDI Preference tab
- Check if the same thing happens in other GB Projects
- If possible check if the same thing happens with a different DAW
- If possible check if the same thing happens with a different MIDI keyboard
The general consensus online seems to be the dual layer/zone thing, but the [poorly written] technical manual didn't cover the Casio's features, so no way to determine if that might be the case here..
I guess you could do a search for MIDI monitor utilities, maybe there's a free one out there that still works.. Note that Logic [along with many other DAWs] shows the incoming MIDI data, and has a quick command for deleting duplicate notes, and larger DAWs often have options for filtering out duplicates [either on input or after the fact]..