"The following control(s) already exist in the Brows Rig Group. Controls can only belong to a single Rig Group."
In Retargeter Character Setup, attempting to add controls to a group that already contains at least one of the selected controls. You may only have a particular control in one of the groups.
When adding controls during Character Setup, make sure that none of the controls already exist in another group.
For more information about Character Setup in Retargeter, see the following article: Character Setup: Part 1 - Control Setup