We are using the SVN polling option for Jenkins. It is set to poll the SVN repository once a minute.

The polling log states:

Started on Feb 7, 2012 11:07:02 AM
Received SCM poll call on for n-oa on Feb 7, 2012 11:07:02 AM
https://XXXXXX is at revision 19,322
(changed from 19,321)
Done. Took 0.25 sec
Changes found

Under "Changes" on the page for the given build, there is the following:

Revision: 19321
No changes.

But then the job never performs the SVN update on the workspace, building from the old codebase. The next poll detects the change, updates the workspace, and builds again. This produces 2 builds for some commits.

We have not been able to determine what triggers the "double build", because, though it is happening often, it doesn't happen every time.