In resharding recipient service, add invocations of collMod for each index that needs to be made unique:true. Specifically, for each such index:

• Execute collMod with the index option “prepareUnique: true” to enforce that any new incoming inserts or updates do not introduce non-unique documents.
• Execute collMod with index option “unique: true”. If this fails due to CannotConvertIndexToUnique, then the whole resharding operation must abort.

The collMod invocations should be performed by the recipient shards just before they notify the coordinator of entering “strict consistency” stage. This is because once the recipient shard notifies the coordinator of entering “strict consistency” stage, it commits to completion of resharding.