Add check for duplicate object member to json

#7182 brought this to my attention, and I remember another previous PR with similar bugs.

I added the proper checks to json.cpp. It will now reject json files that have objects with duplicated members.

Exceptions are made for members named `comment` and members named `//`. Both are already used for comments and therefor it should not matter.

Mandatory link to json standard: http://tools.ietf.org/html/rfc7159#section-4