Enable Log Miner Oracle 10g

I have a user that called me this morning and thinks that the database has actually inserted duplicate rows into his table. I've never used the log miner function.

I cannot stop and restart this database

I'm getting the error when trying to use logmnr
The device is not ready.
ORA-06512: at "SYS.DMBS_LOGMNR"

What do I need to do to view the archived logs?

thanks.

http://www.oracle-base.com/articles/8i/LogMiner.php

if your undo is large enough use Flashback Versions Query or Flashback Transaction Query