It's possible to define a shard key collation that does not match the collection collation. The shard key collation is used for routing, then the _id collation is used for uniqueness.

When $merge-ing into such a collection, we should check that these collations match the source collation. After SERVER-117493, we are checking _id. In this ticket, we will check the shard key.

Even after this ticket, there may still be a race where the shard key is modified concurrently. That's out-of-scope for this ticket. It seems pretty infeasible to address that case.