Fix for #1445 - remove some duplicate dubbed video mappings, allow some others.

After chatting with Bilal, we can keep some duplicates (a single dubbed video ID can be used to replace multiple english IDs), and should remove others (when a dubbed video ID is actually an English youtube ID).

This PR takes care of that, then makes the production system more robust, to avoid any issues in the future.

Note that Bilal has now removed the duplicated IDs that we shoudl remove, so you'll have to trust me that I tested this :)