12 hours? Definitely. That's only about half a degree away in our orbit around the sun. The impact wouldn't be long in coming.

6 months though? Then it would be exactly on the opposite side of the sun, in our L3 point.

It was actually once speculated that there might be a "Mirror Earth" there, but it wouldn't be a particularly stable location, and our probes have long since confirmed there's nothing significant there.