if there is a unique index on field other than _id.

This is because the resharding oplog batch applier partitions a batch of oplog to apply by _id of the document to each worker thread. This creates a scenario where ops are executed on a different order compared to the source collection, potentially introducing a uniqueness violation that never occurred in the source collection.

Some examples (unique on field x):
op0: insert _id: 1, x: 1
op1: delete _id: 1
op2: insert _id: 2, x: 1

Example order that cauases a uniquness violation: op0 -> op2 -> op1

Another example (unique on field x):
op0: insert _id: 1, x: 1
op1: insert _id: 2, x: 2
op2: update _id: 2, set x = 3
op3: update _id: 1, set x = 2

Example order that causes a uniquness violation: op0 -> op1 -> op3 -> op2