If we have the tag search there would not ba a problem with the repeats. Plus, we need an index with the number of usages. This way, we can weed out duplicates and manage it effectively.
Personally, I think we should go with tags that are like this, Zombie(s), so that it works with the plural and singular use. Moreover, it is better to use English words, since Asian words are different because of spelling and new drama fans might not understand it.
So, my proposed fix is that:
1. We add the tag search.
2. Add a tag index.
3. Choose what to use, the plural or singular.
If the problem is not fixed after that, we can look for a different fix. But for now, it is not a problem to allow users to add custom tags, the problem is that there is no way to search and know if it exists.