Commented by: @WarmBooter

I hear this for years: Mixing DML and DDL in the same transaction is ask for trouble :-(
FB should deny such situations since it seems that mixing both operations in the same transaction is not "safe". A better option, of course, would be to fix the problems, but I'm not sure if this is possible.

Commented by: @pavel-zotov

This is author's slightly modified script; checked on 2.1.7, 2.5.9, 3.0.6, 4.0.0 - result the same. It will create PK and UNIQUE for table with fully duplicate rows.

set bail on;
shell del c:\temp\tmp4test.fdb 2>nul;
create database 'localhost:c:\temp\tmp4test.fdb';
show version;

set autoddl off; -- [ !! ]

commit;
set echo on;
recreate table test(
a int not null,
b int not null
);

commit;

insert into test(a, b) values (1,1);
insert into test(a, b) values (1,2);

commit;

-------------------------------------------------------
update test set b = 1;
alter table test add constraint test_unq unique (a);
-------------------------------------------------------
commit;

-------------------------------------------------------
update test set a = 1;
alter table test add constraint test_pk primary key (b);
-------------------------------------------------------
commit;

show table test;
show index test;

select * from test;
commit;