If you do a diff on 19w36a.json & 19w35a.json you can see that 2 new subsections were added to the downloads section called "client_mappings" and "server_mappings". Those two are most likely the culprit for the duplicate key error.
So this 'bug' of requiring an additional txt file was introduced with snapshot 19w36a on 2019-09-04 and it also affects release 1.14.4 because it was added retroactively to that version on the same day. That's not how such a thing should be handled normally, imho the next higher version should've introduced it to the release branch. Because of this it actually does sound more like an anti piracy feature to me.
Just to add my 2 cents:
If you do a diff on 19w36a.json & 19w35a.json you can see that 2 new subsections were added to the downloads section called "client_mappings" and "server_mappings". Those two are most likely the culprit for the duplicate key error.
If anyone is curious enough you can find links to those files @ https://launchermeta.mojang.com/mc/game/version_manifest.json
So this 'bug' of requiring an additional txt file was introduced with snapshot 19w36a on 2019-09-04 and it also affects release 1.14.4 because it was added retroactively to that version on the same day. That's not how such a thing should be handled normally, imho the next higher version should've introduced it to the release branch. Because of this it actually does sound more like an anti piracy feature to me.