@pvarley I have an update for this topic.
I downgrade couchbase version to 4.5 and there is the same issue again.
I noticed that after I swap/reblance or add a node to our cluster and rebalance, duplicated results showed up.
And I have also found another issue, same as this one
I am not sure how to reproduce it, but I suspect that there is something to do with adding node/fail-over node.
Do you have any idea of this issue?