Mongo shell showing following warning

WARNING: the collection 'n.user' lacks a unique index on _id. This index is needed for replication to function properly
2015-08-17T19:09:29.298+0530 I STORAGE [initandlisten] To fix this, you need to create a unique index on _id. See http://dochub.mongodb.org/core/build-replica-set-indexes

We have added a new secondary member (version 3.0) to an existing replica set with three members (version 2.6). Why this collection lacks indexes ?

and mongo suggests to create index by stopping secondary and creating index by running it in a new port. How much time this takes to build new index and if add this member to replica after index creation, will it sync automatically ? if this process takes more time than oplog length(17 hrs) what are the complications ?