Hi, is it possible to lay a second layer of resin onto a fully cured floor (3-5mm). There are small areas where the resin is a bit thin and wondered if it's possible re-coat these areas with small amount of resin to thicken it up a bit. Or is it a case of having to pour another whole area over the whole floor?....it's about 1.5 metres square? Thanks

in my opinion sanding doesn't work, if you are trying to keep the pennys. if epoxy isn't cured properly, sanding usually leads to getting all the dust rubbed into the epoxy, so your penny floor optics are lost anyway.
It sounds like a major mixing problem. if there are areas, wich are still liquid, all you can do is to apply some heat, and hope for the best. if it gets reasonably hard, maybe a second layer (well mixed) will give the needed surface hardness. but for that, the first layer have to be solid .
if not, you should get new pennys, and somehow get the old epoxy with the pennys out. this might include hammer and chisel.