The system will require you to enter a unique registration number when adding the details of a company car record. We recommend that you use a slightly different registration number when adding the details of a replacement car, e.g. use ABC1(1), ABC1(2), ABC1(3) etc. instead of the actual number of ABC1. Alternatively, use the "Auto generate" button which create a unique system generated reg number. Either of these options will work as the HMRC do not require or use actual car registration numbers.
Manual unique reg number