BUG: read_json silently skipping records?
should raise when `orient='columns'` and index is non_unique
`orient='index'` and column is non_unique?

I'm trying out to_json and read_json on a data frame with 800k rows. However, after calling to_json on the file, read_json gets back only 2k rows. This happens if I call them in series or if I give to_json a filename and call the filename with read_json. Judging by the size of the file, all the data is being written (the json is roughly the size of the pickled data frame object). Any idea what's going on?

![image](https://f.cloud.github.com/assets/611122/856362/0f8d00f8-f53b-11e2-8af9-265886f7fc03.png)
