Role of weak entities in avoiding inconsistencies
Definition and example of weak entity

The diagram shows a strong entity (Order, key Order number) linked to multiple weak entities (Order Line, key Order number+Order linenumnber). There's a foreign key from the weak entity to the strong (Order number).

So the Order number is duplicated in the sense a given Order number appears in many entities: one strong (Order number is the whole key); many weak (Order number is part of the key).