What database are you using for the alpha version with "experimental_feed": true? I just tried a clean install of version 1.11.8a2 and didn't run into any issues. Do you have a full prodigy.json file you can share?
For 1.11.7 (or 1.11.8a2 with "experimental_feed": false since it'll run the same code) we don't have a concrete fix for the duplicate examples issue. My only recommendation would be decreasing the batch_size. We're planning to fix forward and move users to the new Feed and Database implementations as soon as possible.
Thanks for your patience on this issue.