Actually, the duplication issue is bug 62321, which is currently marked WORKSFORME.
What people need to do is open support cases and provide more data to the developers. The workaround, if you're faced with this bug and you have to use Lion, is to use CardDAV, but if you do then you need to be prepared for some quirks involving contact groups at the moment.
Please vote for the bug. Also please submit a support request; these are not counted against your support request quota if they're for a bug.
But really, if you have Lion, I'd strongly suggest ditching iSync and setting up IMAP, CalDAV, and CardDAV manually.
No, currently Zimbra doesn't support Exchange accounts. Note: don't confuse "Exchange ActiveSync" as used by mobile devices with Exchange Web Services. See Bug 38631 - Enable snow leopard messaging features, Outlook for Mac 2011 for details.