create table inctest (id int identity( 1 , 1 ), name char( 10 ))
go
declare @i int
set @i= 100 
while @i >  0  begin
    insert inctest (name) values( 'line ' + cast(@i as char( 3 )) )
    set @i = @i -  1 
end
go
delete inctest where id between  5  and  95 
go
delete inctest where id >  98 
go
declare @id int
set @id = isnull((select max(id) from inctest), 1 )
dbcc checkident (inctest, reseed, @id)
go
insert inctest (name) values( 'new line 1')
insert inctest (name) values( 'new line 2')
insert inctest (name) values( 'new line 3')
go
select * from inctest order by  1 
go

 --- results ---
 
id          name       
 ----------- ---------- 
 
 1 . 00         line  100   
 2 . 00         line  99    
 3 . 00         line  98    
 4 . 00         line  97    
 96 . 00        line  5     
 97 . 00        line  4     
 98 . 00        line  3     
 99 . 00        new line  1 
 100 . 00       new line  2 
 101 . 00       new line  3 

( 10  row(s) affected)

 -- посмотрим, что есть в таблице
 
select * from inctest
go

 --- results ---
 
id          name       
 ----------- ---------- 
 
 1 . 00         line  100   
 2 . 00         line  99    
 3 . 00         line  98    
 4 . 00         line  97    
 99 . 00        new line  1 
 100 . 00       new line  2 
 101 . 00       new line  3 
 96 . 00        line  5     
 97 . 00        line  4     
 98 . 00        line  3     

( 10  row(s) affected)

 -- выберем существующий ID перед дырой
 
select min(id) as id from inctest
where id not in(select id- 1  from inctest)
go

 --- results ---
 
id            
 ----------- 
 
 4 . 00 

( 1  row(s) affected)

 -- если дыра
 
set identity_insert inctest on
insert inctest(id,name) values( 5 ,'заполнили')
set identity_insert inctest off

 -- иначе
 
insert inctest(name) values('добавили')