Fredo is right, of course - but I think one should explain why this has to be done:
In the “old” days of mixing consoles, we had a master out and an independent monitor out. The master out was connected to the master recorder (1/2 inch tape, anyone? 
…), the monitor out to the speakers (and nothing else).
This setup would be possible perfectly with Nuendo, too - but as most of us are using the main out for monitoring reasons only (especially when mixing ITB), this seemingly “wrong” double routing occurs.
Kind regards,