The normal sequence of events during resync or first time activation and sync is to wait for the reconcile to finish and then restart Post. This solution explains what happens if the post is restarted without waiting for the reconcile to finish.
If Post is restarted without waiting for the reconcile to finish, this will result in duplicate messages not removed from the Post queue by the reconcile process. These messages are considered duplicate since they have already been applied to the target by way of recovery using archive logs from the source. The net result will be an out of sync condition due to duplicate records applied on the target, if no unique constraint exists for the tables in question on the target. If the tables have unique constraints enabled, then there will be no problems and the Post will only show ORA-0001 (unique constraint violated) but that will just be a false out of sync since the constraint will ensure that no duplicate rows are applied on the target.