How to update GridView Record Based on Unique column value using ASP.Net

see sir in above gridview image, there are some columns, in that two columns one is person Name and another one is User Name, here the condition is the personName and username should be unique, see what i want to do here is while updating the row if the person name or username already exist for same branch code, then i has to show message like username/person name already exist if the personName or username is doesn't exist for same brach then i has to update.