CREATE TABLE [TestTable] (
	[Name] [nvarchar](10) NOT NULL,
	[ID] [uniqueidentifier] NULL
)
go
INSERT [dbo].[TestTable] ([Name]) VALUES (N'Alice')
INSERT [dbo].[TestTable] ([Name]) VALUES (N'Bob')
INSERT [dbo].[TestTable] ([Name]) VALUES (N'Alice')
INSERT [dbo].[TestTable] ([Name]) VALUES (N'John')
INSERT [dbo].[TestTable] ([Name]) VALUES (N'Jane')
INSERT [dbo].[TestTable] ([Name]) VALUES (N'Jane')
GO

update TestTable set ID = newid();

;with T as (
	select Name, ID, row_number() over (partition by Name order by rand()) as _rn
	from TestTable
)
update T1 set ID = T2.ID 
from T T1 join T T2 on T1.Name = T2.Name and T2._rn = 1 and T1._rn > 1;

;with T as (
	select *, first_value(newid()) over (partition by [Name] order by rand()) as new_id
	from TestTable
)
update T set ID = new_id