He repeats here, by way of exaggeration, what he had said James 3:9, to show how exceedingly absurd it is that two such contrary actions should proceed from the same agent. These things ought not so to be; there is a meiosis in the words; he means, things should be quite contrary. See the like expression, 1 Timothy 5:13 Titus 1:11.
Related Commentaries of James 3:10
James 3:10
10 Out of the same mouth proceedeth blessing and cursing. My brethren, these things ought not so to be.