Resolving the MOSS 2007 error: An item with the same key has already been added.

Problem
Starting a Parallel Approval workflow returns an error and produces an item in the Event Log:

Exception occurred during request processing. (User: [HOST]\[username], Form Name: , IP: 192.168.0.1, Request:
http://myServer/_layouts/IniWrkflIP.aspx?List={GUID}&ID=1&TemplateId={061ED225-F894-4E66-BE89-796B6F332E8C}&Source=/Pages/default.aspx, Form ID: , Type: ArgumentException, Exception Message: An item with the same key has already been added.)

Solution
Clear the browser's cache.