Evening - Do Ring sessions guarantee any kind of uniqueness, or will they repeat over time?

Do you mean the cookie for in-memory server-side storage? If so, you shouldn't get collisions - the cookie is generated using java.util.UUID/randomUUID (see https://github.com/ring-clojure/ring/blob/master/ring-core/src/ring/middleware/session/memory.clj)

No then there shouldnt be collisions. Thanks @chrisjd