It can only have an oxygen atmosphere if it's developed life. Without plant and animal life in balance, we'd have a methane atmosphere.
But even further back along that food-chain, if we didn't have the moon, we wouldn't have waves to keep the oceans churned up, nor would we have tide-pools to concentrate the ingredients of life, all of which our current understanding of the way life formed on Earth, require.
It may be the same SIZE as the Earth, but it may share NONE of the other attributes that have developed over the past 3.5 billion years.
The development of life on Earth was a looooooong shot. And without life, it's likely to be a bare rock, if it exists at all.
Let's not forget that Mars is only marginally smaller than the Earth and it didn't have enough gravity to hold in it's atmosphere, and hence lost its oceans and possibly it's life as well.
There are a billion reasons a planet can fall apart from what we consider Earth-like.
Remember, it is only possible. Nobody has seen anything in that expected location as yet.
The development of life on earth was a one for one, not a looooong shot ... it didn’t happen by accident. Too many variables ALL fell into perfect place and at the exact right timing.