Trust you to be different, Gizmonster!
Title numbers are meant to be uniquely allocated to properties, so that two properties can't share a title number and two title numbers can't be allocated to a property. All that I can think of here is that someone has used the wrong form when transferring title to a property and that hasn't been picked up by the Land Registry.
Properties that haven't changed ownership for many years can still be unregistered with the Land Registry. When they do change hands though, they then have to be registered. If someone believed that your property wasn't already registered, when it actually was, they would have applied for a new registration. If the Land Registry also couldn't see that there was already a registration in place, they would then have issued a new title number for the duplicate registration that they'd erroneously created. That might explain what's happened in your case.
Use the web form here to query the situation with the Land Registry:
https://customerhelp.landregistry.gov.uk/contactus
Alternatively try a phone call to 0300 006 0411. (Mon to Fri, 0800 to 1700. Calls are charged at the same rate as those to 01 and 02 numbers, on both landlines and mobiles. If you've got spare 'included minutes' in a phone package, that means that your call will actually be free).