Template compiler should report duplicate reference variable declarations

[chalin]

Duplicate template reference variable "declarations" can lead to confusing results. It would be great if the new template compiler would report this as an error. E.g.,

<input #name>
...
<input #name>

cc @kwalrath

[matanlurey]

Technically this is OK, if you nest views, FWIW.

We could make sure you don't do this in the same view, though.

/cc @alorenzen