Check if the version has been already released

At the moment it is possible to release the same version twice which I guess is not very good.

I added version check which will halt the release if the same version is already there.

I think version rewriting may be ok (not sure though), but (1) it should require some extra flag (e.g. `--force` as in `git push --force`) and (2) it should rewrite the existing version, not make a new one.

The changes are not so huge as it seems. I had to move JSON reading and parsing earlier to avoid creating files and stuff.