The ghosting is almost certainly caused by having too slow of a shutter speed. It's important to understand that when you are dealing with flash, you are potentially making two exposures at the same time, one for the flash and one for the ambient light.
Think of it this way, if you shot in a pitch black room with your flash, 100% of the exposure would come from the flash itself which has a very short duration. You could in theory (and in practice) shoot with a 30 second shutter speed and still get tack sharp photos, assuming you managed to focus in the dark. Only for those few milliseconds that the flash is firing, do photons accumulate on the sensor.
Now think of normal, non-flash photography. In broad daylight, if your shutter speed is, oh 1/8th of a second with a 50mm lens, you are probably going to see some camera shake, or subject movement, or both. The reason for this is the ambient light continues to accumulate over the duration of the exposure, and anything moving in the image will blur. In this case, the answer is to change your ISO or open up your aperture until you can get your shutter speed to roughly the reciprocal of your focal length, ie 1/60th for a 50mm lens, or 1/30th for a 35mm lens. (though you can usually coax an extra stop of hand-holdability out of the shake reduction feature, but only with inanimate subjects)
When mixing flash and ambient indoors, you want enough ambient light to make the photo look like the room is lit naturally, but not enough that it causes your subjects to blur. If a compromise needs to be made, and this will be determined by the amount of ambient light, then it's usually best to underexpose the background by a stop to maybe a stop and a half, and let the flash add the missing light to your subject. So take a look at the exposure info on some of these shots where you see ghosting. What was your shutter speed? Can you afford to open the aperture or raise the ISO a stop, and increase the shutter speed by the same amount? (n.b. this is easiest to do in manual mode) Let us know how it goes.
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