Double stack entries with post_mortem

I can't figure out what is going on here. If you try [jedi](https://github.com/davidhalter/jedi), and use `./sith.py random code --pudb`, where you replace `code` with some random Python code (like try PuDB's own code base), the stack items appear twice (note, jedi is highly recursive, so the stack traces will tend to be big and repetitive).

You'll need to use my [pudb](https://github.com/davidhalter/jedi/pull/273) branch, which fixes pudb integration.

I've attached two screenshots showing what I mean. Notice how there are two `>>` in the stack trace.