track-by property raise warning (and is not honored) when using binding

[noirbizarre]

When using a binded track-by, a warning is raised and the directive is not honored.

See this JsFiddle: http://jsfiddle.net/noirbizarre/d1pr9vq1/2/
In your console, you will read: [Vue warn]: Duplicate track-by key in v-repeat: undefined.

It prevent from coding parametric list components.

Reproducible on Vue.js 0.12.7 and 012.8 (only tested on them so far)

[noirbizarre]

Thanks !