Okay, I'm reopening this thread due to updated scientific developments. Although I didn't think of it at the time I originally contributed, there is one possible circumstance where 'counter earths' could temporarily exist- but I'd have to stress the strictly temporary and relatively ephemeral nature of this co-existence. When solar systems such as ours originally form, there are more than just a relatively small number of protoplanetary 'embryos' present and as a consequence, interplanetary space is more chaotic and congested than is the case later in its history. Some of those embryonic protoplanets end up crashing into others, resulting in the disintegration of some of those bodies if they're roughly the same size or (as occurred with the Earth and the Moon) formation of relatively large planetary satellites from the debris of any prior collision if it was a suitable mass. In the case of the gas and ice giants within our own solar system, smaller protoplanets might have been eaten by them or become their satellites if they were in their orbital vicinity. Due to gravitational turbulence if planets are in comparatively close orbital tracks, it's also possible that some planets got either flung out of our solar system altogether and are currently rogue planets, or into eccentric orbits (which may explain why our solar system has no superearth large terrestroid planets- it's possible the elusive "Planet Nine" might be one- or one theory goes that originally there might have been a fifth gas giant before it got ejected from our solar system when Jupiter and Saturn lay closer together, given their high planetary mass).

Thus, any Counter-Earth would not have had a particularly stable or permanent orbital trajectory and wouldn't have stayed on exactly the opposite side of the Sun for long. Doctor Who gets it right when it comes to Mondas, the home world of durable series villains the Cybermen, which seems to have begun as a Counter-Earth before it drifted away (or was ejected?) from our solar system.

Why is this relevant now? Well, as some of the scientifically interested individuals amongst the rest of you might have figured out, there has been a sighting of what may be two exoplanets sharing almost the same orbital track- PDS 70 b appears to have a dwarf protoplanet beginning to form at one of its Trojan points. PDS 70b and its apparent satellite/co-orbital dwarf planet, as well as a further gas giant, PDS 70c,   orbit a young variable T Tauri star (only about ten million years old) around 370 light years away from our solar system. If this is further independently validated and this isn't just a quirk of the imaging technology used, then the explanation that I've suggested above may be probable- PDS 70's solar system may have been too young to get rid of all of its surplus embryonic exoplanets as yet, and we may be witnessing the formation of  a large planetary satellite akin to our own or a temporary state of affairs that may end in an eventual planetary collision similar to the one that formed our own Moon. However, given the size of PDS 70b (estimated to be larger than Jupiter), it'll probably be a light snack. (Did I say "Jupiter"? That whole scenario sounds a lot like the mythological Greco-Roman god Cronos/Saturn, who was also in the habit of eating his offspring, until Zeus/Jupiter escaped and liberated his siblings.)

To conclude then, any Counter-Earth in the Captain Marvel story outlined above would therefore have to have been an intruder alternate Earth which was introduced into its solar system by a malevolent third party (possibly the Anti-Monitor or a Universe-S cosmic scale villain), if Universe-S has laws of physics, electromagnetic and gravitational constants comparable to our own. If that is the case, any Counter-Earth could not have survived undetected in a stable orbit exactly matching our own orbital momentum for a prolonged period, especially when Earth became able to send interplanetary probes to Mercury, Venus or Mars.

Source: en.wikipedia.org/wiki/PDS_70