Error in pass2 for type FSharp.DataFrame.FSharpFrameExtensions, error: duplicate entry 'Frame`2.GroupRowsBy' in method table

[from fsbugs] 

Simplified repro:

``` fsharp
type Frame = 
  class 
  end
module X =
  type Frame with
    member frame.GroupRowsBy(key) = ()
    member frame.GroupRowsBy(key) = ()
```

Issue: Signatures in extrinsic type extensions are not validated for uniqueness.
