I'm not sure if this helps explain the flow, but this is how things work for me.
I add a new calendar event to my BB -> it sync's to Google Calendar -> it then sync from Google to Outlook.

Next If I add a new event to Outlook -> it then syncs to Google -> then from Google to my BB.

Finally If I add a new event to Google Calendar -> it syncs to Outlook and to BB.

I do not get duplicate entries.
I also do not use BES, I am a BIS only user, So I do not know if BES adds another layer to the mix.

I figured out that there are different calendar views and the one that I thought was 2 of the same appointment was actually the "all" calendar view. Once I selected Work only I saw only one appointment.

Unfortunately it doesn't sound like the GooSync is going to work for me. I was trying to find a way to get my Outlook onto my personal BB. If I could do that I wouldn't have to always have the BES one with me. Although...maybe if I can figure out how to get the whole thing over ONCE, then any updates that are made or additions, will add....hmmmm

Has anyone tried exporting the existing entries from Outlook calendar and then importing them into Google calendar? I'm just wondering if this would work or just lead to duplicates (ie, once you import them into Google, the sync would see them as new posts and no longer existing posts, therefore syncing them again with the BB)?