How to skip the first and consider the second row whenever duplicate cells come in a column SQL Server -

i have following table. startdatetime column repeated when duration on day. want write query skips first row , count difference of second row whenever duplicate cell in startdatetime column come. e.g. in case of following sample table output want given in table.

create table test     ([name] varchar(50), [startdatetime1] datetime, [enddatetime2] datetime, diffy int) ;  insert test     ([name], [startdatetime], [enddatetime2], [diffy]) values     ('abc', '2015-07-21 16:08:02.000', '2015-07-21 16:18:10.000', '608' ),     ('abc', '2015-07-21 16:18:10.000', '2015-07-21 23:06:46.000', '24516' ),     ('abc', '2015-07-21 16:18:10.000', '2015-07-23 12:37:35.000', '159565' ),     ('abc', '2015-07-23 17:33:35.000', '2015-07-24 11:07:00.000', '63205' ) ;   ╔══════╦════════╗ ║ name ║ diffy  ║ ╠══════╬════════╣ ║ abc  ║    608 ║ ║ abc  ║ 159565 ║ ║ abc  ║  63205 ║ ╚══════╩════════╝ 

the simple query used

select   name,  datediff(second, startdatetime, enddatetime) diffy  test 

i think want group by:

select name,        datediff(second, startdatetime, max(enddatetime)) diffy test group name, startdatetime;