Jerusalem, she had been the LORD's, yet was recompensed double for her spiritual whoredoms:

Jeremiah 16:18 And first I will recompense their iniquity and their sin double; because they have defiled my land, they have filled mine inheritance with the carcases of their detestable and abominable things.

Isaiah 40:2 Speak ye comfortably to Jerusalem, and cry unto her, that her warfare is accomplished, that her iniquity is pardoned: for she hath received of the LORD’S hand double for all her sins.

Mystery, Babylon will also be rewarded double for her spiritual whoredoms:

Revelation 18:6 Reward her even as she rewarded you, and double unto her double according to her works: in the cup which she hath filled fill to her double.

So, is part of the mystery of Mystery, Babylon, that she had been the LORD's, before she, like Jerusalem, turned away unto whoredoms?