Is there a requirement that an activity ID be unique?

Reason why I ask is it seems prudent to save a list of encountered activities, and drop those that have been seen before.

However that caused me to run head-first into a NodeBB regression because we ourselves don't actually send unique activity IDs.

For example, a Follow-Undo(Follow)-Follow chain would have the two Follows with the same ID, since we just construct them ad-hoc based on request data.

Easy fix is to throw in a timestamp there, but it got me wondering about whether there were uniqueness expectations at all, or whether I was being overzealous in checking for it.