In some cases, it's possible for two labels to be created with the same name (even though we do checks for this). It can occur when there are two parallel requests (via the web or XML-RPC), which try to add the same (non-existent) label. Since they're in separate transactions, it's not possible to see that another session is trying to create the label, so both end up in the DB.

Possible solutions to this:

Write a periodic task that merges any duplicate labels (and the affected content)
Isolate the sections of the current transaction in the save process, to narrow the window in which this can occur
Add some sort of read/write filtering to the transaction so the changes can be seen before the transaction ends (??? - Scott's suggestion)