the ordinary numeral adverb signifying twice, is rendered "again" in Philippians 4:16 , "ye sent once and again unto my need," and in 1 Thessalonians 2:18 , where Paul states that he would have come to the Thessalonians "once and again," that is, twice at least he had attempted to do so. See TWICE.
the regular word for "again," is used chiefly in two senses, (a) with reference to repeated action; (b) rhetorically, in the sense of "moreover" or "further," indicating a statement to be added in the course of an argument, e.g., Matthew 5:33; or with the meaning "on the other hand, in turn," Luke 6:43; 1 Corinthians 12:21; 2 Corinthians 10:7; 1 John 2:8 . In Hebrews 1:5 palin simply introduces an additional quotation; in Hebrews 1:6 this is not so. There the RV rightly puts the word "again" in connection with "He bringeth in the firstborn into the world," "When He again bringeth, etc." That is to say, palin is here set in contrast to the time when God first brought His Son into the world. This statement, then, refers to the future second advent of Christ. The word is used far more frequently in the Gospel of John than in any other book in the New Testament.