If duplicate users exist then the upgrade process of creating a new unique index will fail. To fix this duplicate user problem you must start in the previous version and ensure that no duplicate users exist before upgrading; this will require removing those duplicate user documents.

Orig Report
Example from original thread on mongodb-user.

After upgrading to 2.4 I have following errors:

Wed Mar 20 10:38:42.006 [initandlisten] build index admin.system.users { user: 1, userSource: 1 }
Wed Mar 20 10:38:42.007 [initandlisten] exception in initAndListen: 11000 E11000 duplicate key error index: admin.system.users.$user_1_userSource_1  dup key: { : "myuser", : null }, terminating