I am not familiear with All Sync, so I can only offer general advice.
Duplication normally happens when you have a mishmash of syncing going on, like Outlook syncing to Google, the iPad syncing to the computer, etc. Instead set things up directly. Choose on source an sync every device directly to that source.
If you have any contact syncing set up in iTune's Info settings, turn that off.
Unless All Sync offeres you some advantage to syncing your iPad directly to Google (like maintaining Groups/Lables, I'd abandon it. You can set your
Google account up as an Exchange account to sync contacts. Or if you've done this and are also using All Sync, then be sure to turn off the contact syncing in the account on the iPad.
Anyway, I think you'll get it. You have to make absolutely sure that there is only one path to each device to sync. No loops, branches.
Hopefully that was not too confusing.