Display the latest row only - sql

Question

Goal:
If you retrieve any duplicate data that is data in the column secondid, then you to retrieve one row only from the latest date. For instance in the data below I have two different datetime, I would like to retrieve the data '2016-05-02 07:34:14.377' from value 6 in column secondid.
Problem:
I code seems not to be working and what am I missing.
Info:
There are many data in and you cannot hard code the value in in the code.
CREATE TABLE [dbo].[testing2](
[id] [int] NOT NULL,
[secondid] [int] NULL,
[value] [varchar](30) NULL,
[category] [int] NULL,
[test_id] [int] NULL,
[id_type] [int] NOT NULL,
[Testing2Datetime] [datetime] not NULL,
CONSTRAINT [PK_testing2] PRIMARY KEY CLUSTERED
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
INSERT INTO [test].[dbo].[testing2]
VALUES (3, 3, 'a' ,2 ,11 ,1, '2016-05-01 07:34:14.377');
INSERT INTO [test].[dbo].[testing2]
VALUES (4, 4, 'a' ,2 ,11 ,1, '2016-05-01 07:34:14.377');
INSERT INTO [test].[dbo].[testing2]
VALUES (5, 5, 'a' ,2 ,11 ,0, '2016-05-01 07:34:14.377');
INSERT INTO [test].[dbo].[testing2]
VALUES (6, 6, 'a' ,2 ,11 ,2, '2016-05-01 07:34:14.377');
INSERT INTO [test].[dbo].[testing2]
VALUES (7, 6, 'a' ,2 ,11 ,2, '2016-05-02 07:34:14.377');
select
bb.secondid,
max(bb.Testing2Datetime)
from [dbo].[testing2] bb
group by
bb.secondid,
bb.Testing2Datetime

Answer 1

The maximum value of Testing2Datetime per Testing2Datetime is the Testing2Datetime itself. You should remove it from the group by clause and retrieve the maximum value per secondid only:
select
bb.secondid,
max(bb.Testing2Datetime)
from [dbo].[testing2] bb
group by
bb.secondid -- Here!

Answer 2

Remove bb.Testing2Datetime column from Group by
select
bb.secondid,
max(bb.Testing2Datetime) as [Max_Testing2Datetime]
from [dbo].[testing2] bb
group by
bb.secondid
or even this(Row_Number window function)
select *
from
(
select
bb.secondid,
bb.Testing2Datetime,
Row_number()over(partition by bb.secondid order by bb.Testing2Datetime desc) as RN
from [dbo].[testing2] bb
) A
Where RN = 1