Thanks for your help, but I think there is an error here. I believe you definitely can restore a snapshot on an existing index as long as it's `closed`.
The restore operation can be performed on a functioning cluster. However, an existing index can be only restored if it’s closed and has the same number of shards as the index in the snapshot. The restore operation automatically opens restored indices if they were closed and creates new indices if they didn’t exist in the cluster.

So I think, the question remains: if I do this, what set of data will be in the cluster afterwards.

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