Gracefully handle malformed deduplication key in Redis

What does this MR do and why?

!101925 (merged) changed the serialization format of the deduplication key in Redis from a standard string to a MessagePack-encoded string. If you had upgraded from a previous version and ran tests, it's likely that the current format can't be unpacked by MessagePack. To deal with this situation gracefully, just delete the key and consider it empty.

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