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There are 46 questions using passive and more than 300 questions using passive-voice.
Should not the tags be merged? I would make passive-voice the main tags, and passive its synonym.
Yes, I agree. There's no reason to maintain these as separate tags. They both clearly refer to the passive voice, and there is no distinction in how the tags are used.
I've gone ahead and merged passive into passive-voice, just as you've suggested.