ink incorrectly allows redefinitions of CONSTs

(Originally posted by @joningold on our internal Asana.)

The following is valid and shouldn't be:

```
CONST PI = 3.1
CONST PI = 3.14
{PI}
```

We quite like the ability to re-define `CONST`s to the **same** value (for importing the same file in multiple places), but perhaps this is a hack to get around the fact that `INCLUDE` statements are like C-style copy/paste, and could perhaps be revised to automatically only include once.