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I don't know for certain with Sony cameras, but I imagine that the first flash actually fires before the shutter opens and is a pre-flash to measure the flash strength so that it can be set for the exposure proper. You notice it more easily with rear sync because there's the length of the exposure between the two flashes.
If you want to capture background detail lit by whatever ambient light there is, slow or rear sync is the way to go. Moving objects that are patly ambient & partly flash lit tend to look better with rear sync.
Chris
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