Dear Fleur, so you observed the same with double-forms. Exactly, the one that I always notices is on Spetses island, right outside Klimis café. Oleanders are all hightly toxic, but I suppose sniffing is not dangerous.
They are planted here (Attica, Greece) everywhere lately, and allowed to coppice unchecked. Yes, I definitely prefer this scented double-form, especially if diligently pruned to form a multi-trunc small tree.