I'm not sure whether they're biologically unique (as fingerprints are not), but the way standard scanners work has both false positive matches and false negatives. NIST maintains benchmarks for biometric errors, which are usually in the tenths of a percent range.
I guess you should hope none of your hundreds of thousands of iris neighbors are placed on the list?
I guess you should hope none of your hundreds of thousands of iris neighbors are placed on the list?