I am curious if uniqueid value is always unique in asterisk, I hope it is unique on given asterisk server but can dupliactes be created with ihgh voilumes?
On a given Asterisk it is unique, and across multiple Asterisks it can be made unique by configuring a server identifier which is used as part of it.
However, it uniquely identifies a channel and a channel may have more than end point associated with it during its life and result in more than one CDR. You cannot use it as a unique ID for storing CDRs in a database.