Double negatives, e.g., "I can't get no satisfaction," are termed informal intensifiers and are used to stress negation. The rules affecting English grammar are not synonymous with mathematical operations, so it is an error to believe that two negations must create a positive. Of course, they can, if that is the speaker's intent:
"Mr T doesn't look as if he can't kick
some butt." - Eddie Murphy
By the same token, a double positive would work as an intensifier, strengthening the degree of emphasis, as in this review of a Burgundy by critic Clive Coates:
Good balance. Positive at the end.
Very good indeed.