Is it correct to write
This often is done for something.
This is often done for something.
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My thought that the variant "This is often done for something" sounds more grammatically correct.
Because the adverbs of frequency have to precede the main verb. Actually the adverb is put before the word it's modifying.
But your alternative variant "This often is done for something" definitely make sense, though it sounds a bit as a colloquial expression.