Posting this since I've already written it
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If(OBJECT_ID('tempdb..#temp123') Is Not Null)
Begin
Drop Table #Temp123
End
Create Table #temp123 (
C_SS_KEY nvarchar(13)
,DS_Start_date Datetime2
,DS_End_Date datetime2
,CTNumber nvarchar(7)
)

insert into #temp123 values('614-P085199-P', '2017-07-28 16:15:15.0000000','2017-09-19 23:00:05.0000000','P085199')
insert into #temp123 values('614-P085199-P', '2017-09-19 23:00:05.0000000','2018-02-12 19:00:48.6300000','P085199')
insert into #temp123 values('614-P085199-P', '2018-02-12 19:00:48.6300000','2018-02-15 19:00:48.6300000','P085199')
;WITH BASE_DATA AS
(
  SELECT
   T.C_SS_KEY
  ,T.DS_Start_date
  ,T.DS_End_Date
  ,T.CTNumber
  ,MONTH(T.DS_End_Date) AS MONTH_NO
  FROM #Temp123 T
)
SELECT
  BD.C_SS_KEY
 ,MIN(BD.DS_Start_date) AS DS_Start_date
 ,MAX(BD.DS_End_Date) AS DS_End_Date
 ,BD.CTNumber
FROM  BASE_DATA BD
GROUP BY BD.C_SS_KEY
   ,BD.CTNumber
   ,BD.MONTH_NO
;

Output

C_SS_KEY  DS_Start_date     DS_End_Date      CTNumber
------------- --------------------------- --------------------------- --------
614-P085199-P 2017-09-19 23:00:05.0000000 2018-02-15 19:00:48.6300000 P085199
614-P085199-P 2017-07-28 16:15:15.0000000 2017-09-19 23:00:05.0000000 P085199