One area we've seen this is after several users have requested software via the Software Portal, sometimes from the same machines.
By design, multiple threads work the TaskInstanceRequest list. When a thread moves on to a new item, it first checks to see if it has been processed by any other thread. If it has, then it skips that task and the warning you are seeing is logged. We plan on dropping the log severity of these messages down to 'informational' in SP2.