"Double" as adjective: is it attributive-only, never predicative?

I can't find authoritative endorsement here, but I'll give a reasoned answer.

(1) As an adjective, 'double' is usually used attributively. I can't find an example sentence in a dictionary where it is used predicatively.

(2) One can readily find well-written examples on the internet of the form

• Prescription Drug Benefit: If a person chooses to buy two policies, the benefits are double.

However, all these seem to be reductions of

• ... the benefits are double (= twice) what they would otherwise be.

(3) I'd say that 'Melville's achievement was double (= twofold)' is at best unusual. Here, supporting evidence (not proof) is available, from Google ngrams:

[About half the hits for 'benefits are double' are false-positives, with strings like 'the benefits are double the pre-modernisation ones' or compounds like 'the benefits are double-edged'.]