Blackberry/Outlook mail multiplier

I have a couple of users running Outlook that also have Blackberry's.

The problem they are experiencing is receiving multiple copies of emails on their desktop machines.

Is there a need in Outlook to leave the messages on the server so that the Blackberry will get them or will un-checking this option result in only a few messages making it to the Blackberry?

 

I always configure the user's pda to leave a copy on the server otherwise they won't be able to view them on their PC. I have also noticed on a few devices (we don't use just one brand here) that if they have their outlook open when not in the office, the phone will not get emails

(Our email is configured to reside on the PC's, not the server)