'Double-minded man' is an apposition, not with 'that man,' but with 'he that doubts' (ver. 6); and ver. 7 is practically a parenthesis. I do not think the sense of ver. 8 bears a direct connection with ver. 7; it is rather a moral explanation of the figure of ver. 6. The style of James is characterized by these proverbial sentences: see ver. 20 as an example.