Check with the individual and try the old ID Number, or maybe even a surname.
I have had that message displayed if there is a difference in the name. For example "de Bruyn" vs "de Bruin". It gets confusing because in one instance I had the name correct, but is existed on another profile with the name incorrect. In the end I had to register with the incorrect name, request tax type transfer, and then amend details to the correct name.

It also happens with companies. If a company was registered for tax on a specific name, and after it underwent a name change you try to register it on a profile with the new name, that exact message is displayed.