I understand that the same user will have different page-scoped IDs (PSIDs) across different pages. But is it guaranteed that no two users will have the same PSID across different pages?
For example, is it possible that user A has PSID 1234 while messaging page X, and user B has the same PSID 1234 when messaging page Y?
Technically user A and user B would be differentiable even if they have the same PSID because they are messaging different pages. But can I assume that there will be no overlap on PSIDs across all pages, so that I may use the PSID as a globally unique ID regardless of the particular page?
Facebook don't guarantee the page-scoped IDs (PSIDs) to be unique across different pages - a PSID may represent user A on page X, but user B on page Y.
When representing a user, these IDs are page-scoped IDs (PSID). This means that the IDs of users are unique for a given page.
https://developers.facebook.com/docs/messenger-platform/webhook-reference#format
来源：https://stackoverflow.com/questions/41192810/are-facebook-messenger-page-scoped-ids-globally-unique