Jamieson, Fausset and Brown on James 1:8

8. double-minded--literally, "double-souled," the one soul directed towards God, the other to something else. The Greek favors Alford's translation, "He (the waverer, Jas 1:6) is a man double-minded, unstable," &c.; or better, Beza's. The words in this Jas 1:8 are in apposition with "that man," Jas 1:7; thus the "us," which is not in the original, will not need to be supplied, "A man double-minded, unstable in all his ways!" The word for "double-minded" is found here and in Jas 4:8, for the first time in Greek literature. It is not a hypocrite that is meant, but a fickle, "wavering" man, as the context shows. It is opposed to the single eye (Mt 6:22).