It does this in one of various ways, but the most common method is simply to store a list of GUID's that it has already displayed and "marked as read" or similar.
Ọ bụ otu n'ime ọtụtụ ụzọ, ma usoro kachasịsịsịsị iche bụ nanị ndepụta nke ndị GUID na o gosipụtaworị na "akara dị ka a na-agụ" ma ọ bụ yiri.