I'm not that fast a typist, but I seem to remember having such an issue with MorphOS a while back, though I can't remember whether it was isolated to my MacMini, or my A1138 PowerBook (like you have) . . . or even if it was back 4 or so years ago, in my older Amiga days (which I still have, but currently don't use).

I'm not sure what cured it either, I'm just offering zoi some support that I've noticed this double letter or multiple character input even though I felt sure I wasn't pressing the key that was still triggering the input. I thought it was just the hardware then, as I don't think anyone else has mentioned this issue until now, though I could be mistaken.

However, I don't think I've had that issue for a while, but then I usually use my Apple Bluetooth (A1016) keyboard with my Mac Mini for MorphOS, which works really well. Though I also have a few A1048 USB keyboards as well, but can't remember having that issue with them either, well not recently that I can remember, even though I've used them recently, but that was often installing a Mac OS X for someone else.