@Thomas:
Well... you could be right, never mind actually
The A1200 motherboard have the input for matrix keyboard. Matrix connected to microcontroller placed on the MB (a plcc44 quadratic chip with (M) [motorola] logo). It converts matrix keystrokes further to the amiga serial keyboard protocol, which then goes to the 8520 chip serial port.
There are 2 ways connecting external at=ps/2 keyboard converters - to the matrix instead of real keyboard, the converter then must emulate keypresses, OR to the serial pins of either keyboard controller or 8520 chip (because the serial interface is open-collector and permits parallel connecting of several controllers).
In the second case, no such effect could emerge, in the first case it is quite likely.
The advice is to check, clear, reconnect the cable which should be connected in place of original A1200 keyboard (if any).
