My D750 has a habit of double focusing. By this I mean I half press the shutter and it focuses but then it clicks out of focus and then back in. It almost seems to be focusing twice. It is the same with various lenses but doesn't occur on my D610 with any of the same lenses! Any ideas? Camera has been back for flare problem to be sorted but is not apparently affected by the new issue of shading of half of the image