MongoDB Replica Set member cannot start due "84 key/value already in index"
Do initial sync for that problematic secondary node. Just delete files from dbpath and start mongod. Node will do initial sync automatically. Other and faster way (if you can) is copy snapshot of dbpath of existing node (can be primary or secondary) and start mongod.

The problem here is that there is already entry at unique index and system tries to apply opLog line and crashes to that error.