An individual who appears at a port of entry in Canada is not fingerprinted. They may be fingerprinted at the time of deportation, for example, but as the system is currently built, those fingerprints aren't collected on re-entry.
As to how this has happened in the past, I'm not engaged in that kind of activity, so I don't know all of the details, but I do know that people have used photo-substituted passports, for example, and appeared at a port of entry with a new identity. Altered documents can be quite sophisticated in nature, and the fraud can be quite difficult to detect. Something like biometrics, especially linked to a reliable database such as might be kept by the RCMP, really increases the integrity of the system.