The latest launcher update (1.6.51) has brought along a major bug.
When a new .json file gets created, a new "downloads" argument is added. That arranges it so that each and every time Minecraft is launched, all files are downloaded again.
Shouldn't that only happen if files are missing, or if a new Minecraft version is getting loaded?
Whilst this "downloads" argument is still in the .json file, it's impossible to install mods as well, because the Minecraft version gets reinstalled each time Minecraft is launched.
There is a screenshot of a comparison between an old .json file, and a new one, with the "downloads" argument (marked green).

Or is this whole matter deliberated?
If not, please fix it

Kind regards
HuanZon