Report for Duplicate aliases

This is a fix for #139. 

`tests/pos/typeAliasDup.hs` now fails with

```
**** CRASH:  Multiple Declarations Error: tests/pos/typeAliasDup.hs:5:10 *******
          Multiple Declarations of Type Alias `BoundedNat'
          Declared at: tests/pos/typeAliasDup.hs:5:10
                       tests/pos/typeAliasDup.hs:11:10

**** CRASH:  Multiple Declarations Error: tests/pos/typeAliasDup.hs:6:15 *******
          Multiple Declarations of Predicate Alias `Foo'
          Declared at: tests/pos/typeAliasDup.hs:6:15
                       tests/pos/typeAliasDup.hs:12:15
```