Albert Barnes Commentary Revelation 18:6

"Render unto her even as she rendered, and double [unto her] the double according to her works: in the cup which she mingled, mingle unto her double." — Revelation 18:6 (ASV)

Reward her even as she rewarded you. It is not said to whom this command is addressed, but it would seem to be to those who had been persecuted and wronged. Applied to mystical Babylon—Papal Rome—it would seem to be a call on the nations that had been so long under her sway, and among whom, from time to time, so much blood had been shed by her, to arise now in their might and to inflict deserved vengeance (see Barnes on Revelation 17:16-17).

And double unto her double according to her works. That is, bring upon her double the amount of calamity which she has brought upon others; take ample vengeance upon her. Compare, for similar language, Isaiah 40:2, She hath received of the Lord's hand double for all her sins.Isaiah 61:7, For your shame ye shall have double.

In the cup which she hath filled. To bring wrath on others (see Barnes on Revelation 14:8).

Fill to her double. Let her drink abundantly of the wine of the wrath of God—double that which she has dealt out to others. That is, either let the quantity administered to her be doubled, or let the ingredients in the cup be doubled in intensity.