Remove Duplicate based on column value-linq

i have many to many relationship between employee and group. following linq statement

int[] GroupIDs = {6,7};


var result = from g in umGroups
    join empGroup in umEmployeeGroups on g.GroupID equals empGroup.GroupID
    where  GroupIDs.Contains(g.GroupID)                     
    select new {  GrpId = g.GroupID,EmployeeID = empGroup.EmployeeID };

returns groupid and the employeeid. and result is

GrpId  | EmployeeID
6      |   18
6      |   20  
7      |   19
7      |   20

I need to remove the rows for which the employeeid is repeating e.g. any one of the row with employeeid= 20
Thanks