While updating a document, query by range return duplicate documents

Encounter a very odd situation. Anyone meet it before?

For example, there is 1000 documents in a index.

While my program is updating documents (by docid), i want to get all
documents in this index. I do such by query by range (from x size y) in
one loop. Finally, i get the right number documents (size is 1000),
but in documents there are many duplicate documents (same document).

At last, have to restart the cluster and do the same step above, The issue

Wonder what cause this weird situation?

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