Jamieson, Fausset and Brown on James 1:24

24. beholdeth--more literally, "he contemplated himself and hath gone his way," that is, no sooner has he contemplated his image than he is gone his way (Jas 1:11). "Contemplate" answers to hearing the word: "goeth his way," to relaxing the attention after hearing--letting the mind go elsewhere, and the interest of the thing heard pass away: then forgetfulness follows [Alford] (Compare Eze 33:31). "Contemplate" here, and in Jas 1:23, implies that, though cursory, yet some knowledge of one's self, at least for the time, is imparted in hearing the word (1Co 14:24).

and ... and--The repetition expresses hastiness joined with levity [Bengel].

forgetteth what manner of man he was--in the mirror. Forgetfulness is no excuse (Jas 1:25; 2Pe 1:9).