13:2a I already warned you the second time I was with you.

When I visited you (plur.) the second time, I warned those who sinned earlier and all the others.
-or-
During my second visit, I warned those who sinned in the past and all the others ⌊there⌋.

13:2b So now in my absence I warn those who sinned earlier and everyone else:

Now while I am not yet there, I repeat what I said beforehand,
-or-
Before I come ⌊there⌋, I now warn again ahead of my next visit ⌊to you⌋

13:2c If I return, I will not spare anyone,

that if I come back there I will not let any of them escape punishment.
-or-
that when I return I will not have mercy ⌊on any who have not repented⌋.

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