This is a curious one, partly because the idea of taking time to decide on a suitable question tag to follow the apparently urgent utterance of, “Someone is coming” is a little odd! I would tend to say, “Someone is coming! Quick – hide!” or whatever else is more relevant to the situation! As a UK native English speaker, I’m inclined to agree with Elaine Milner, that, “aren’t they?” is the only appropriate question tag.
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