Any question answered...

Is it possible that there is an undiscovered second earth planet on the earth's orbit, just mirrored 180°, so it's impossible to spot from here - as it would always be hidden by the sun?

Or would Voyager space probes and the likes definitely have discovered it already?

That would be the http://en.wikipedia.org/wiki/Lagrangian_point known as L3. Nothing there sorry.