If you mean on the opposite side, in the same orbit(in the earth-sun L3), this has been hypothesised as far back as the presocratic Greeks(https://en.wikipedia.

If you mean on the opposite side, in the same orbit(in the earth-sun L3), this has been hypothesised as far back as the presocratic Greeks(https://en.wikipedia.org/wiki/Counter-Earth https://en.wikipedia.org/wiki/Counter-Earth), but we now have a lot of evidence that it isn't there. Also, planet 9 was hypothesised in order to explain the orbits of transneptunian objects, which an antiearth wouldn't do. If you mean behind the sun on a completely different orbit, well it wouldn't stay behind the sun relative to us forever, and likely would only very rarely be obscured by the sun.