Revelation 18:6; In the Cup With She Hath Filled to Her Double

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. 

The cup is the same one mentioned earlier in Revelation 14:8; 10; 17:4; 18:3.

Revelation 14:8  And there followed another angel, saying, Babylon is fallen, is fallen, that great city, because she made all nations drink of the wine of the wrath of her fornication. 

Revelation 14:10  The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb: 

Revelation 17:4  And the woman was arrayed in purple and scarlet colour, and decked with gold and precious stones and pearls, having a golden cup in her hand full of abominations and filthiness of her fornication: 

Revelation 18:3  For all nations have drunk of the wine of the wrath of her fornication, and the kings of the earth have committed fornication with her, and the merchants of the earth are waxed rich through the abundance of her delicacies. 

The vessel Babylon used to seduce others has now become the instrument of her own punishment (Lee). The instructions to her executioners filled fill to her double in that cup. This is a re-emphasis on doubling for her wrongdoings. The same verb “I pour out or mingle” tells of the mixing of undiluted divine anger in Revelation 14:10.

[Robert L. Thomas, Revelation 8-22 An Exegetical Commentary, Moody Press, 1995, 324]