A.T. Robertson 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)

Render as she rendered (αποδοτε ως απεδωκεν). Second aorist (effective) active imperative and first aorist (effective) active of αποδιδωμ, old and common verb for requital, to give back, the lex talionis which is in the O.T. (Jeremiah 50:15,29; 51:24,56; Psalms 137:8), and in the N.T. also (Matthew 7:2). Here the reference is to persecutions by Rome, particularly the martyrdom of the saints (18:24; 19:2).

Double the double (διπλωσατε τα διπλα). First aorist imperative of διπλοω, old verb (from διπλοος, double, Matthew 23:15), here only in N.T. Διπλα is simply the neuter plural accusative (cognate) contract form for διπλοα (not διπλω). Requite here in double measure, a full requital (Exodus 22:4,7,9; Isaiah 40:2; Jeremiah 16:18; Jeremiah 17:18; Zechariah 9:12). The double recompense was according to the Levitical law.

Which she mingled (ω εκερασεν). First aorist active indicative of κεραννυμ. The relative ω is attracted to the locative case of its antecedent ποτηριω (cup), for which see 14:8,10; 17:4; 18:3.

Mingle unto her double (κερασατε αυτη διπλουν). First aorist active imperative of the same verb κεραννυμ, with the same idea of double punishment.