... they are only guaranteed to be unique ...
[I'm assuming this was a typo ... ?]
Uniqueness is only *guaranteed* if there's a unique, one-column index that consists of the identity column.
There's nothing *technically* keeping someone from inserting multiple rows with the same identity value, or updating multiple rows to have the same identity value. Examples include using 'set identity_insert', 'set identity_update', and resetting the max/next identity value to a lower value thus allowing ASE to re-issue identity values.