SQL Left Join first match only

distinct is not a function. It always operates on all columns of the select list.

Your problem is a typical “greatest N per group” problem which can easily be solved using a window function:

select ...
from (
  select IDNo,
         FirstName,
         LastName,
         ....,
         row_number() over (partition by lower(idno) order by firstname) as rn 
  from people 
) t
where rn = 1;

Using the order by clause you can select which of the duplicates you want to pick.

The above can be used in a left join, see below:

select ...
from x
  left join (
    select IDNo,
           FirstName,
           LastName,
           ....,
           row_number() over (partition by lower(idno) order by firstname) as rn 
    from people 
  ) p on p.idno = x.idno and p.rn = 1
where ...