I am duplicating with vertices an empty that is duplicating a group. In Cycles (and possibly in BI), it is rendering the this dupligroup base object in both render preview (expected) and in the final render. The documentation specifies that the base object should not be rendering which would make sense as otherwise this feature becomes completely useless and the lack of this feature is a serious problem.
When the object being duplicated with verts is an empty that itself is duplicating a group, in Cycles only, the empty is rendered as well which is incorrect behavior.
In BI, the group that the empty is duplicating is not rendered either when the empty is being vertex duplicated which is again incorrect behavior. As expected, if the empty is duplicating a group and is itself not being vertex duplicated, the group is rendered.