Not sure which Outlook version you're using, but Outlook 2000 for me had some problems. It doesn't seem to compare names directly, but has some other way of telling entries apart. So syncing 2 computers and my BB was a pain.
I ended up getting one Outlook all straight, and deleting everything on my phone. Then I synced that Outlook to the phone. Still having problems on the 2nd computer with the same name not always syncing, but creating a new entry. My plan is to back up that one, delete all the names and then sync from the phone.