Constant REPEATABLE_PROPERTIES 

pub(crate) const REPEATABLE_PROPERTIES: PropertyIdSet;

Property IDs that may appear more than once in a properties section.

Everything else is a Protocol Error when repeated (§2.2.2.2). Treating Subscription Identifier as repeatable everywhere is slightly lenient: the spec only allows multiples in a delivered PUBLISH (§3.3.2.3.8), not in SUBSCRIBE, but a per-packet distinction is not worth the complexity for a client that never receives SUBSCRIBE packets.