Flightcrew not reporting error when the toc has several navPoints with the same playOrder

```
Flightcrew reports "No problems found!" on an epub file that has an invalid
.ncx file. E.g., a file where two navpoints have the same playorder will
validate even though this is invalid structure for the .ncx file.

Since this is easy enough to verify, I do not attach a test file
```

Original issue reported on code.google.com by `royevena...@gmail.com` on 4 May 2012 at 4:33

Start by locating the NCX validation entry point and the handling of navPoints and playOrder. Reproduce the report with an NCX file containing two navPoints with the same playOrder, then add coverage showing that Flightcrew reports the invalid structure instead of "No problems found!".

Written by the indexing model from the issue text.