Lots of reasonable answers here, but I think the simplest approach is to sanitize the data in an INSTEAD OF trigger, which will fire before you actually insert the data. That way your AFTER triggers only fire once because you only modify the table once. EG

create table t(id int primary key, a varchar(20))

go

create or alter trigger t_II on t
instead of insert
as
begin
  insert into t(id,a) 
  select id, trim(a) from inserted 
end

go 

create or alter trigger t_AI on t
after insert
as
begin
  select 'after trigger running'
end

Lots of reasonable answers here, but I think the simplest approach is to sanitize the data in an INSTEAD OF trigger, which will fire before you actually insert the data. That way your AFTER triggers only fire once because you only modify the table once. EG

drop table if exists t
go
create table t(id int primary key, a varchar(20))

go

create or alter trigger t_II on t
instead of insert
as
begin
  set nocount on

  print 'instead of trigger running'
  insert into t(id,a) 
  select id, trim(a) from inserted 
end

go 

create or alter trigger t_AI on t
after insert
as
begin
  set nocount on 

  print 'after trigger running'
end

go

insert into t(id,a) values (1, '  a  '),(2, '  a  ')

outputs

instead of trigger running
after trigger running

(2 rows affected)