Rather than failing with a 'shard already exists' error, addShard should return success if a shard with the same shard name and replica set name already exists. This makes addShard idempotent, which in turns makes it safe to be automatically retried on network errors.
- related to
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SERVER-24214 Validate shardIdentity document of proposed shard when adding a shard that already exists
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- Backlog
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