This is weird if you don't have USB connected.
Have you tried to use MIDI-OX or similar to see whether also a PC receives multiple MIDI Note On per key pressed?
My keyboard has a Harmony mode that can create echoes, trills etc that's even reflected over USB (echo is done by sending multiple MIDI On with diminishing velocity, etc, quite cool actually

). The CTK seems to have something similar. Maybe that got activated.