Actually, I was initially incorrect. This isn't just a problem with encryption. Within a single GetUpdates request, we're guaranteed to never see the same item twice. However, when syncer.cc loops and makes multiple GetUpdates requests, and then combines those results, before we get to apply/merge, we do need to handle receiving both the same update twice, and separate updates for the same item.

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Already implemented in DeduplicatePendingUpdatesBasedOnClientTagHash().