This sort of structure is called a double negative, but in effect it shows affirmation.

He's got a dual personality - usually a quiet "nice guy" type, but when he flips, his character changes.

The canal can accommodate ships as large as 150,000 dead weight tons.

I see double.

Beauty is but skin deep.

Beauty is but skin deep.

He has a dual personality.

Age, like distance, lends a double charm.

This word has a double meaning.

Arnold is concerned with cases of dual personality.

Lenses with two distinct optical powers are also called "bifocals".

This word has a double meaning.

I actually have a split personality.

Tom has a split personality.