Impossible for a sequence generator (Oracle object) to hand out the same sequence twice.
If it could, it would be a serious bug. And a known bug.
As this is not the case, and you have failed to provide any evidence to the contrary, the logical reaction is that you have a bug in your code - and you are misdiagnosing that bug as the same sequence reusing the same number more than once.
In addition to Billy:
You should always provide your full database version. ( the result of: select * from v$version; )
There's a known issue with sequences on multi-node RAC:
If you have access to Metalink/MyOracleSupport, then do a search there as well.
But I'd put my $0.02 to Billy's reply and check your code.