In ArcMap, create a Unique Values symbology, select multiple categories, then right-click and Group them.
Publish to ArcGIS Server and add as a dynamic map service. Using the new JS API version 2.1 legend widget, create a legend from this layer.
I'm finding that the grouped categories display multiple times. Eg, if the group consists of 3 individual categories, the (identical) symbol will appear in the legend 3 times.
Please let me know if you need any further information to reproduce this bug.
Thanks,
Steve
PS I have no idea why this title has a smiley face. Maybe that's another bug.