We're also experiencing this problem following an upgrade from Zimbra 5.0.10 to 6.0.2 on Ubuntu 8.04 LTS 64-bit (zcs-6.0.2_GA_1912.UBUNTU8_64.200910201516200). A shared address book with 1036 entries yields duplicate results. A test [shared] address book with <20 addresses yields normal, expected results. I can also attest to the problem about abnormal CPU usage that occurs when duplicate contact entries are displayed. Load average jumps from about 0.20 to 1.20+ when accessing the shared address book with 1036 entries.
I see the bug severity level was bumped to "Critical," so I'm praying for a patch or fix release soon
In the interim, I exported the host account's address book to a CSV file and made it available to users requiring it (not a lot of them, but enough to be inconvenienced). Unfortunately, this will create an unnecessary amount of duplicate information on the server, but it's better than angry users.