Albert Barnes on Revelation 18:6

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. Rev 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, Isa 40:2, "She hath received of the Lord's hand double for all her sins." Isa 61:7, "For your shame ye shall have double."

In the cup which she hath filled. To bring wrath on others. Barnes on "Re 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.

(i) "Reward her" Ps 137:8, Jer 50:15,29