with a (numb, val1, val2) as (values
  ( 1 , 'v11', 'v21')
, ( 2 , 'v21', 'v22')
, ( 2 , 'v21', 'v23')
, ( 2 , 'v22', 'v24')
, ( 3 , 'v31', 'v32')
)
, b(rn, numb, val1, val2) as (
select rownumber() over(partition by numb) rn, numb, val1, val2
from a
), c (rn, numb, val1, val2) as (
select rn, numb, cast(val1 as varchar( 128 )) val1, cast(val2 as varchar( 128 )) val2
from b
where rn= 1 
  union all
select b.rn, b.numb
, c.val1 || case locate(' '||b.val1||' ', ' '||c.val1||' ') when  0  then ' '||b.val1 else '' end val1
, c.val2 || case locate(' '||b.val2||' ', ' '||c.val2||' ') when  0  then ' '||b.val2 else '' end val2
from c, b
where b.numb=c.numb and b.rn=c.rn+ 1 
)
select c.numb, c.val1, c.val2
from c
join (
select numb, max(rn) rn
from c
group by numb
) g on g.numb=c.numb and g.rn=c.rn
order by c.numb