Sequence.nextval doubles the returned value with Execute Statement (F9)

Question

There appears to be a quirk with sequences in Raptor.
Has anyone noticed that depending on how you execute this sql (SELECT MYSEQ.NEXTVAL FROM DUAL;) the value returned is either the correct nextval or double what you expected?
For example, MYSEQ is a simple sequence which increments by 1. If you Execute Statement (F9) then the value returned jumps by 2 instead of 1. If you Run Script (F5) then the value returns jumps by 1, as expected.
If MYSEQ is changed to increment by 2. The when you Execute Statement (F9) then the value returned jumps by 4 instead of 2. If you Run Script (F5) then the value returns jumps by 2, as expected. No matter what you put for the increment by Execute Statement (F9) always doubles it.
It always seems to be double. Executing the same scenario in TOAD always returns the correct value (i.e. properly increments).
Is the query being executed multiple times with Execute Statement? Why is this happening?

Answer 1

While there is no guarantee from Oracle that sequences produce sequential numbers, this is obviously a case where SQL Developer is running the select statement twice.
The issue is that queries can actually change information, rather than just retrieve data from the database.
The following package is a test case:
create or replace package test_query is
function get_next_count return number;
end;
create or replace package body test_query is
cnt number := 0;
function get_next_count return number is
begin
cnt := cnt + 1;
return cnt;
end;
end;
select test_query.get_next_count from dual;
This query, which should return 1, 2, 3, 4, etc actually returns 2, 4, 6, 8, etc, because SQL Developer is running the select twice.