Where there are contacts in Xero with secondary "people", the Xero API will duplicate them when syncing invoices.
People will continue to be duplicated until it reaches the limit of 5, resulting in a Wise-Sync validation error.
What we're doing about it
Xero have been made aware of this issue and are currently investigating a solution.
We're currently investigating the possibility of limiting the amount of contacts returned to us by Xero, to prevent duplication on sync.
If you have any questions, please contact us at firstname.lastname@example.org.