detect key conflict in a JSON file

[Roy Smith]

In article <mailman.2347.1369826248.3114.python-list at python.org>,
 Jabba Laci <jabba.laci at gmail.com> wrote:
 
> I have a growing JSON file that I edit manually and it might happen
> that I repeat a key. If this happens, I would like to get notified.

The real answer here is that JSON is probably not the best choice for 
large files that get hand-edited.  For data that you intend to hand-edit 
a lot, YAML might be a better choice.

> Currently the value of the second key silently overwrites the value of
> the first.

Yeah, that's what I would expect.  http://www.json.org/ is mute on the 
question of what to do with duplicate keys, but I would be quite 
surprised to discover any implementation which behaved differently.

> Do you know about a command line JSON validator?

All JSON implementations validate their input.  You are assuming that 
having duplicate keys is "invalid".  I would think it falls more into 
the category of "undefined behavior".