1.)
According to most threads i've read today, you cannot delete mobs from mongen if they have a duplicate entry in the db.
What i tend to do is Export as Excel Spread Sheet - Edit - Import back into SQL..
2.) If you have messed up your Mongen.db and are trying to start again, Delete mongen table, Then use the data.mdb files thats in most server files to Restore the db.
Hope that helps, ...