By looking at the code it seems that this may be possible under the following circumstances:

• A request to create an issue is received and gets the relevant project from the Project Manager.
• The Project Manager refreshes its cache of Projects.
• A second request to create an issue is received and gets the new Project object.
• Now, the two separate instances of Project can try to use the same key.

This is all dependant on concurrent operations happening in a particular sequence, and would be more likely to happen on a JIRA server that was under high load.