Jamieson, Fausset and Brown on 2 Corinthians 13:2

2. Rather, "I have already said (at my second visit), and tell you (now) beforehand, AS (I did) WHEN I WAS PRESENT THE SECOND TIME, SO also NOW in my absence (the oldest manuscripts omit the 'I write,' which here wrongly follows in English Version Greek text) to them which heretofore have sinned (namely, before my second visit, 2Co 12:21), and to all others (who have sinned since my second visit, or are in danger of sinning)." The English Version, "as if I were present the second time," namely, this next time, is quite inconsistent with 2Co 13:1, "this is the third time I am coming to you," as Paul could not have called the same journey at once "the second" and "the third time" of his coming. The antithesis between "the second time" and "now" is palpable.

if I come again, &c.--that is, whensoever I come again (Ac 20:2). These were probably the very words of his former threat which he now repeats again.