Hi RMerrill, thanks for posting on the Community
We'd like to make sure that you're able to reinstate your employee in QuickBooks Payroll.
The system is designed to flag when duplicate employee details are entered. As a workaround, you can edit the national insurance number on the original profile (for example, change the last digit) and then create the new employee profile with the correct NI number.
If you continue to encounter an issue, please get back in touch below with the version of payroll that you're using (Core or Advanced). We'll be here if you need any support.