This validation error may occur when copying invoices to Xero if a contact created by CaseFox in Xero was deleted in Xero. Xero has the habit of caching deleted entries and not allowing same identification numbers to a new contact.
Here is the proposed solution.
1. Find the casefox client whose invoice is throwing this
2. Change its name by adding --- next to first and last names
3. Create a new client record same as before (without using --- next to first and last names)
4. Now click on the old client record, then on the right side click on the case name.
5. Click on Edit Case tab of the case and then click on "Move this case under another client" and move it to the new client record.
This will give the client a new identification and will work in Xero.