Select list query requires two columns: one is DISPLAY value, another one is RETURN value.
Usually, it is something like
select department_name as display_value,
department_id as return_value
from depts
Sometimes, the source table's contents is such that the same column represents both display AND return value - just like in your case. An example might be this:
SQL> with currency (code) as
2 (select 'EUR' from dual union all
3 select 'USD' from dual union all
4 select 'GBP' from dual union all
5 select 'CHF' from dual
6 )
7 --
8 -- Apex query would then be
9 --
10 select code as display_value,
11 code as return_value
12 from currency;
DIS RET
--- ---
EUR EUR
USD USD
GBP GBP
CHF CHF
SQL>