查找重复项并更新T-SQL

I have the following table

我有下表

TABLE GrantedService(
 GUID [uniqueidentifier] NOT NULL PK,   
 ServiceTypeGUID [uniqueidentifier] NOT NULL,
 ServiceOrderGUID [uniqueidentifier] NULL,
 ValidFrom datetime
 ValidTo datetime
 Description [varchar](max) NULL,
 ExternalID [varchar](40) NULL
)
TABLE Grant