Recently a client came into a Service Desk and presented two New Zealand Passports for certification. The passports had different passport number but were in the name of the same person. The two passports were valid and both were active. (ie not cancelled) When queried the client said he travelled a lot.
The Department of Internal Affairs was called to enquire if this was legal and apparently yes it is. If you travel frequently and have lots of visas for other countries you can apply to have a second concurrent passport. Both copies of the passports were certified as they were valid documents.
I personally was aware that you could hold two passports but one was your original and the other one was a diplomatic passport. However, as they say, you learn something every day. Thanks to Jocelyn F for posting.