Olympus E-PL5 / 60 2.8 Macro - double image with IS

I've only had the E-PL5/ 14-42 for a few days and yesterday I was doing head shots of some kids at my school. Same bright shade light, same background. A-mode at f/5.6, with shutter speeds ranging from 1/80 to 1/125. I.S. 1 turned on. Face recognition enabled. (Next time I'll go with "face and eyes.")

On shot review on the computer, zooming up to see the eye detail: some pics were razor-sharp on the eyes. Others... a bit off, but still usable.

For me, I need to test it more thoroughly but will be keen to find the shutter speeds/ I.S.combos to avoid.

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Barry