Dubbing animations and movies is much harder than dubbing documentaries.
Is it necessary to repeat dubbing here?
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As TRomano pointed out, "more harder" is ungrammatical (though frequently seen, sometimes as humor).
Repeating "dubbing" is important, in the form of the sentence you give, because it's not completely clear without re-reading that it's dubbing documentaries that's hard. TRomano's rephrasing would be a good way to avoid repeating "dubbing" if that's what you want to do.