Compiler should check for duplicates in record's property name

In elm 18, you can create a record like this:

```elm
type alias State =
    { title : String
    , forms : Forms.State
    , forms : Forms.State
    }
```

this will work on all modern browsers except IE 11 and below. It will generate the following Javascript  run time error:

"Multiple Definitions of a Property Not Allowed in Strict Mode".

