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Try to review a revision that has already been approved, e.g. on testwiki,

action=review&revid=180749

We see

{
    "review": {
        "result": "Success"
    }
}

but in reality, nothing was done. 

Perhaps some kind of error message is needed here.

Technically nothing went wrong (the revision you wanted to review was reviewed), so an error wouldn't make sense here. I think adding a 'nochange': '' key would work, just like how action=edit works.