Print better compiler error for duplicate identifiers

```ts
class Foo {
  field: i32;
  field: i64;
}
const foo = new Foo();
```
But this should be valid:
```ts
class Foo {
  field: i32;
  static field: i64;
}
```

Expected compiler error:
```
Dublicate identifier 'field'
```
Actual:
```
ERROR: AssertionError: assertion failed
```