Commented by: @hvlad

The problem occurs when records with same keys fall into separate sort run's and that run's not merged until
final phase of index creation (fast_load)

Test case:

SQL> create database 'core-5161.fdb';
Server version:
WI-T4.0.0.32400-dev Firebird 4.0 Unstable
SQL> create table t (id int);
SQL>
SQL> set term ^;
SQL> execute block as
CON> declare i int = 0;
CON> begin
CON> while (i < 100000) do
CON> begin
CON> insert into t values (:i);
CON> i = i + 1;
CON> end
CON> end
CON> ^
SQL> select count(*) from t;
CON> ^

            COUNT

=====================
100000

SQL> set term ;^
SQL> insert into t values(1);
SQL> commit;
SQL>
SQL> create unique index t_id on t(id);
SQL> select * from t where id = 1;

      ID

============
1
1

SQL> commit;
SQL> exit;