The role of the conflict stash collections in resharding's oplog application is to handle any transient duplicate _id values. Transient duplicate _id values arise from the lack of a strict ordering across shards for multi-document transaction oplog entries. Any documents remaining in the conflict stash collections after the recipient has applied through all donors' oplog entries represent non-transient (permanent) duplicate _id values and are therefore violations of the global _id uniqueness assumption in sharding.