This means that there is already an existing contact in Xero with same Account Number or Contact Number as the one you are trying to export from Power Diary.
The Account Number / Contact Code can be seen in Xero by editing the contact. The Client Number in Power Diary can be found by looking at the Client details screen.
The most common cause of this issue is that you have archived the Contact in Xero. Review your archived contacts in Xero and you will probably see one in there that matches the details of the Client you are trying to export from Power Diary.
Un-archive this contact and merge them with any other similar ones you may have in Xero, keeping the contact with the Contact Code / Account Number.
If you use the Company Name field in Power Diary and then export invoices for this client, Xero will create a new contact using the Company Name as the Contact Name. In the example below, the company name in Power Diary was ABC Pty. Ltd. and the Client name was Damien Test.
If you later remove the company name from Power Diary and then export more invoices, Xero will attempt to rename the contact to the Client Name. In this example it would change ABC Pty. Ltd. to Damien Test.
If you already have an existing contact in Xero with this same name (Damien Test) the export will fail because Xero will not allow multiple contacts to have the same name. You may see an Export Result message which reads as follows:
"The existing contact with name 'ABC Pty. Ltd.' cannot be updated to use new name 'Damien Test'. There is an existing contact with that name"
The solution is to merge the contacts in Xero, keeping the Power Diary created contact. See below for how to find out which contact was created by Power Diary.
How to tell which Xero contacts have been created by Power Diary.
Contacts in Xero that have been created by Power Diary will have a Contact Code (see the red arrow on the picture below). This is the Client Number from Power Diary. Xero does not allow users to edit this number.