Redefinitions of functions / methods with the same signature are not caught by the compiler and produce an LLVM error

The following example should work due to overloading:

```
Int add(Int x, Int y):
    // ...
Int add(Int x, String y):
    // ...
```

The following example should NOT work, since the signatures are equal:

```
Int add(Int x, Int y):
    // ...
Int add(Int x, Int y):
     // ...
```

But it does. 
(Redefinition of variables is prevented correctly)
