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Be sure you are selecting the Single shooting option in the Drive mode setting rather than the One-Shot AF option in the AF mode setting. When the AF mode is set to One Shot, the camera can still fire continuously. With One Shot AF once focus is achieved the focus will lock until the shutter is released or the half-press is released.

If you are holding it half way, it is a pretty soft press between a half way press and a full press. Personally, I always use mine in one of the continuous drive modes, but I've had times when I've had it go off prematurely while I was adjusting focus or metering as a result of how light the second stage trigger is. If you have a light touch, I'd hazard ...

It could be that there is something underneath the button (like dirt/condensation) that is causing the camera to fire more than once from time to time. I should add that the MkIII has a very sensitive trigger and that I've bumped it several times accidentally and gotten off bursts of shots even in single fire mode. As dpollitt has suggested above, more ...