Running calcyfy with a -u distribution url adds it to the json, even if it already exists

[spikelynch]

If a distribution url is already defined by the json metadata, and I pass one in with the -u flag, the script adds the -u url to the json file and updates it, even if it's the same URL.

If you aren't aware of this behaviour, repeatedly generating HTML using calcyfy means that each new version has another download link.

The script should probably issue a warning if it gets two distribution urls, and also be smart enough to not generate multiple download links if the metadata has duplicates.