The Bugzilla Importer keeps track of which issues were imported by recording Bugzilla key -> JIRA id mappings in the changeitem table. If the user attempts to import a Bugzilla project twice, the second import will print lots of "Issue: FOO-123 already exists. Not Imported" messages and refuse to import the issues twice.

I suppose this prevention of duplicate imports could be considered a feature, but in practice it is just annoying. For instance, say I import a project, then realise the key is wrong. I should be able to delete the imported project, and reimport the issues, but I can't without hacking the database to remove the Bugzilla -> JIRA records in changeitem