Full Idea
Only an end that is also a duty can be called a duty of virtue. ....[385] The necessary ends are one's own perfection, and the happiness of others.
Gist of Idea
A duty of virtue is a duty which is also an end
Source
Immanuel Kant (Metaphysics of Morals II:Doctrine of Virtue [1797], 383 Intro II)
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.188
A Reaction
So virtues are a subset of duties. I don't think an Aristotelian virtue is anything like a duty. A soldier might do his duty, with no virtue at all. An even a Kantian categorical imperative duty can be formed without right feeling or good character.
Related Idea
Idea 21414 The supreme principle of virtue is to find universal laws for ends [Kant]