Unsure of how the data got this way, but several issues have multiple values in the Rank field. Re-ranking them does not replace the value, but adds an additional one.

Sample error in log:

2008-08-05 12:06:19,269 http-80-Processor13 ERROR [issue.customfields.impl.AbstractSingleFieldType] More than one value stored for custom field id 'customfield_10232' for issue 'CLRT-22694'.  Values [1.5105E11, 1.5105E11]

Have not yet tried re-optimizing the Rank field. I expect that will fix the issue. But clearly it is still a problem if it gets this way to begin with and subsequent ranking only compounds the problem.