That probably indicates that there is some other, additional bug (or condition) which is causing two builds. If you e-mail me your config.xml file for the job, or upload it to this bug report, I can compare your job configuration and my job configuration to see if I can identify any obvious reasons why the plugin would see your job as needing to schedule a second build.
The typical reason why a second build is immediately scheduled is that the plugin thinks the "Branches to build" matches two different branches, and both branches have changes. If polling detects multiple branches to build, it will immediately schedule them both.
You could also add the "Force polling using workspace" behaviour to see if that changes the behavior.