Will a sequence return same value for two different sessions?

Question

Is there a possibility that a sequence will return same value to two different sessions when it is referred exactly at the same instance of time?

Answer 1

@Justin... Thanks for your insight; indeed, we too feel this shouldn't ever happen and never heard of it either, but there it is. (No, we haven't logged a TAR yet -- whatever that is -- partly because it didn't occur to us and partly because we only recently came across the issue and sensibly want to do some testing before we cry foul.)
However, the code is pretty straight-forward, much like this (inside a FOR EACH ROW trigger body):
SELECT <seqname>.NEXTVAL INTO <keyvar> FROM DUAL;
INSERT INTO <tblname> (<keyfield>, <... some other fields>)
VALUES(<keyvar>, <... some other values> );
(where <tblname> is NOT the table on which the trigger is fired). This is the only place where the sequence is ever accessed. The sequence state is way below its limits (either MAXVALUE or <keyfield>/<keyvar> datatype size).
In this setup, end users sometimes got an out-of-the-blue SQL error to the effect that uniqueness constraint has been violated -- as I said, we used to have a unique index on <keyfield> -- which leads us to assume that the sequence generated a duplicate key (only way for the constraint to be violated, AFAIK). We released the constraint and indeed, using a simple SELECT <keyfield>, COUNT(*) FROM <tblname> GROUP BY <keyfield> HAVING COUNT(*)>1 got us some results.
Unfortunately, the <tblname> table gets regularly purged by a consumer process so it's hard to trace; now we created a logger trigger, on <tblname> this time, which tracks cases of duplicate <keyfield> inserts... We'll see how it goes.
@Laurent... winks at the CYCLE thing Our sequence is (needless to say) declared as NOCYCLE and the datatype is large enough to hold MAXVALUE.