I've owned a M6 classic before and just purchased a M6 TTL. I've found the rangefinder flare annoying from time to time. It doesn't happend that often but when you're shooting in bright light/into a lightsource it happends once in a while. Try reposition the camera and keep the eye in the middle of the viewfinder.
You can get it upgraded to the MP finder which should be much less prone to flare.