I have tried to repro this on Linux 64-bit DEBUG, and it passes for me. But this has been failing consistently on BUILDBOT since #5764

Linux 64-bit #5764 Aug 29 (rev d4806a96d5e)
Linux 64-bit #5765
Linux 64-bit #5766

http://buildlogs.mongodb.org/Linux%2064-bit/builds/5764/test/replication/drop_dups.js

linux-32 replication : drop_dups.js
Linux 64-bit BUILDBOT replication : drop_dups.js

"assert: [2] != [1] are not equal : A2 : true"
http://buildlogs.mongodb.org/mci_0.9_linux_32/builds/65/test/replication_0/drop_dups.js
http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_32_84ff1694c4bf92f3deef5c013ac9a5b912e6f96f_13_08_29_22_09_07_replication_linux_32